The answer to this question can be found in Act One of this excellent play. As Abigail and Proctor stand over the catatonic Betty, the congregation below begin to sing a psalm from the Bible. However, as the phrase "going up to Jesus" is heard in the psalm, Betty claps her hands over her ears so she is unable to hear the name of Jesus and begins whining loudly. This gradually escalates into screaming. Of course, when the psalm is finished and Betty's father rushes into the room, accompanied by the Putnams, this is seen as further evidence that some kind of witchery has been going on, as Mrs. Putnam makes perfectly clear:
The psalm! The psalm! She cannot bear to hear the Lord's name!
Thus it is that the suspicions of witchery in Salem begin and are fuelled by Betty's response to hearing the name of Jesus.
While the congregation, below, is singing a psalm, John Proctor and Abigail Williams, Betty's cousin, are arguing about their relationship, right in front of Betty's prone body. As the people sing, Abigail is describing the "pretense" in Salem, the way John opened her eyes to the "lying lessons" she'd been taught. As their conversation becomes more tense and angry, it seems possible that Betty is actually responding to the argument between these former lovers, and it just happens to be when the words "going up to Jesus" are heard. An inability to hear the Lord's name was believed to be a sure sign of witchcraft, but we (and Arthur Miller, the author) know that Betty is not witched; in fact, there is no such thing as witchcraft. It was just prior to John's entrance that Betty woke on her own, and she is "frightened of Abigail" (according to stage direction). She accuses Abigail of drinking blood, saying that she "drank a charm to kill Goody Proctor," John Proctor's wife. Therefore, this relationship between her cousin and John is an obvious source of stress and anxiety to her, and she accuses Abigail of lying to her father, the Reverend Parris, about what she did in the forest. For all these reasons, it makes sense to consider the idea that Betty screams as a result of her anxiety about what her cousin has done, as well as her guilt over her own involvement in the woods.