they were doing that night, and her father will put aside her punishment because he is so worried about her.  So, she pretends to be sick; she refuses to eat or speak, and lies there like she is in a coma.  Occasionally, she'll break out in hysterics, flying at the window to try to "fly."  This bizarre behavior works well to distract her father; concerned, he sends for a doctor, and then for Reverend Hale, just in case witchcraft is involved.  As Abigail tells Proctor in Act One, Betty isn't sick, "she took fright, is all."

I hope that those thoughts help a bit; good luck!

A careful reading of the text indicates that there are a number of things wrong with Betty Parris.

The first and most obvious, initially, is her physical condition. When the play opens, we learn that she is lying inert on her bed with her father, Reverend Parris, the local priest, kneeling beside her bed, supposedly praying. Betty is unconscious.

The reverend is clearly in distress and we later discover that he had sent Susanna Walcott to the local doctor, Dr Griggs, to establish whether he had discovered a reason for Betty's condition. It is evident that the doctor had made an earlier visit. Parris is alarmed when Susanna tells him, on her return, that the good doctor, "...cannot discover no medicine for it in his books."

The reverend becomes quite agitated and anxious when Susanna further informs him that the doctor has said,

...he have been searching his books since he left you, sir. But he bid me tell you, that you might look to unnatural things for the cause of it.

The reason for Betty's sickness is clarified when we read in Parris' conversation with Abigail, his niece, that he had caught Betty and her, with other girls, dancing around a fire in the forest at night. He surprised them with his sudden appearance and Betty fainted immediately, probably from sheer terror and shock. It seems that her condition at this point is a genuine reaction brought about by the immense distress she must have felt on being discovered performing a ritual which is taboo in Salem society and which could have serious consequences, especially since her father runs the local parish.

Betty later suddenly recovers when Abigail threatens to beat her and tells her that she has already spoken to her father. Betty is clearly afraid of Abigail and jumps up screaming that she wants to go to her deceased mother. She is so distraught that she threatens to jump out of the window in an attempt to 'fly to mama.'

It is also clear that Reverend Parris has no skill in working with children. The author's notes inform us that:

He was a widower with no interest in children, or talent with them. He regarded them as young adults,...

This informs us of another problem Betty has. She lacks the love and comfort of a mother and has only Abigail to use as a role model. She was largely raised by Tituba, the reverend's slave from Barbados, who most probably also had much influence on her young mind. Furthermore, Betty is clearly afraid of both her father and her cousin, Abigail, whose own parents had been brutally murdered, so when Abigail threatens 'a pointy reckoning' to any of the girls if they speak about what they did in the forest, one can just imagine how frightened Betty must be. 

It is these fears which drives Betty to lie in court and join the other girls to falsely accuse and implicate innocent residents of witchcraft. The girls' shenanigans in court are so convincing that the judges and prosecution are easily led to believe their pernicious testimony.

Ten year old Betty Parris is the daughter of Reverend Parris.  She has some kind of vague undiagnosed illness that is either made up or psychosomatic (in her head). 

The girls, Abigail and Betty, and their friends were dancing in the woods and their father jumped out, scaring Betty so badly that she fainted.  Reverend Parris thinks that they “trafficked with spirits in the forest” (Act 1, scene 1).  He says that there is a “faction” that is trying to drive him from the pulpit and their actions will make things worse.

Apparently, Betty’s illness is either feigned or in her head because when Abigail confronts her she talks about Abigail “drinking blood” she “suddenly springs off bed, rushes across room to window where Abigail catches her” (1.1).  Betty claims that Abigail drank a charm to kill Goody Proctor.

Now look you. All of you. We danced. And Tituba conjured Ruth Putnam’s dead sisters. And that is all. (Act 1, Scene 1).

She threatens them if they tell.  At this point, Betty faints again.  She is either easily frightened or being conveniently ill.

Betty is a perfect example of how the witch-hunting hysteria affected even the youngest citizens.  Whether Betty’s illness is real or not, she has gotten herself in the middle of it.  Abigail’s threats and strange behavior are also examples.

The short answer to your question is this: nothing, though it sure looks like something. Arthur Miller's The Crucible is set in Salem at the time of the infamous witch trials, and everyone knows witchcraft is a serious--and punishable--offense. When the play opens, we are concerned because ten-year-old Betty Parris is lying on her bed, inert, and everyone who is close to her is worried because she will not wake up. 

Her father, Reverend Parris, has sent for a nearby expert (Reverend Hale) on matters of witchcraft. The Putnams come to the Parris home because their daughter is suffering the same malady; they, too, are convinced that witchcraft is to blame. Rebecca Nurse comes and is compassionate, but she believes this is just a "silly season," which all children have occasionally.

Abigail Williams and Parris (her uncle) talk, and we begin to learn the truth. The girls were dancing in the forest last night. When John Proctor comes to the house, Abigail tells him the truth. Betty is acting like this because she does not want to get in trouble for being in the forest last night. 

Abigail smacks Betty around and threatens Betty and the other girls not to tell the truth about last night; they are just to say they were dancing. Betty suddenly comes out of her stupor.

Betty: You drank blood, Abby! You didn’t tell him that!
Abigail: Betty, you never say that again! You will never--
Betty: You did, you did! You drank a charm to kill John Proctor’s wife! You drank a charm to kill Goody Proctor!
Abigail, smashes her across the face: Shut it! Now shut it!
Betty, collapsing on the bed: Mama, Mama! She dissolves into sobs.

The only thing wrong with Betty is that she did something she should not have done, and now she is trying to avoid getting punished for it.

At the beginning of the play, Betty Parris lies incapacitate on her bed and will not respond to her father, which strikes fear in him as he kneels and prays by her bedside. Unlike Ruth Putnam, Betty's eyes are closed and she seems to be in a coma-like state. Whenever Abigail and Reverend Parris command Betty to speak, she acts like she cannot hear them. Mrs. Putnam is astonished when she sees Betty Parris and mentions that their daughter lies with her eyes open. According to Mrs. Putnam, Ruth also walks and acts like she cannot see or hear. When Reverend Parris leaves, Abigail says that she told her uncle everything and Betty darts from her bed towards the window and attempts to jump out. When Reverend Parris returns to his daughter's room, Abigail tells them that Betty tried to jump out of the window when she heard them singing psalms outside, which Mrs. Putnam believes is a sure sign of witchcraft. Overall, Betty Parris behaves like she is in a coma and cannot respond to her father or cousin. She also experiences sudden outbursts, where she attempts to jump out of the window and fly.

The text does not tell too many specifics about Betty Parris's "illness" and her symptoms.  What is definitely told to the audience is that Betty has not moved herself since midnight.  Even somebody that is asleep tends to move in some way.  Symptomatically, Betty seems to be in a coma like state.  She breathes, but in no other way does she respond to the environment around her.

The only other key detail that the reader is given about Betty is that her eyes are closed.  That may seem obvious, but it surprises the Putnams, when they come to visit.  Mrs. Putnam says that her daughter, Ruth, has the same lack of response as Betty.  The difference though is that Ruth's "coma" seems to be an awake coma.  Her eyes are open, and she walks around; however, she doesn't respond to anything.  

Mrs. Putnam: She ails as she must - she never waked this morning, but her eyes open and she walks, and hears naught, sees naught, and cannot eat. Her soul is taken, surely.

Betty Parris is ill, and it seems as though she cannot wake up because her eyes remain closed.  Doctor Griggs is baffled by her condition and can find no physical cause, and so he advises Betty's father, Reverend Parris, to "look to unnatural things for the cause of it."  There is also a rumor circulating, told by Mrs. Putnam when she arrives, that Betty flew "over Ingersoll's barn, and come down light as a bird [...]."  Parris assures everyone, at first, that her illness has nothing to do with witchcraft, as this is what Abigail, his niece, tells him.  She says, "when you leaped out of the bush so suddenly, Betty was frightened and then she fainted.  And there's the whole of it."  However, Parris doubts Abigail and continues to question her about her reputation in town.  

A while later, Betty does wake up and try to "fly to Mama," running suddenly for the window.  She shouts at Abigail that she didn't confess everything to Parris, and she knows that her cousin "drank a charm to kill Goody Proctor."  Thus, because she was able to hear Abigail and awake when she chose, it seems that her "illness" is merely a guilty conscience.

Betty has fallen mysteriously ill.  However, this is really her guilt at having been discovered dancing in the woods with the other girls and Tituba.  Tituba is quiet worried about her Betty, for she is Parris's slave and has practically raised Betty.  Tituba is also likely worried that her mysteriously illness might lead other to wonder what is causing it and that might lead into an investigation into what was going on in the woods.  Of course, it was innocent tomfoolery; however, in the strict Puritan religious culture, it could be construed as witchcraft, which could lead to hangings.  The rumor circulating about Betty is that she is under the spell of witchcraft, which is one reason the community is seeking Hale, who is an expert on witchcraft and leading trials.