By the time Mayella takes the stand, her father Bob Ewell has been pretty much proven to be a liar and a bigot. Even though Atticus was respectful the entire time as he questioned Bob, he still did a very good job of pointing out the evidence that showed that Bob was lying about the entire scenario, and framing an innocent black man for something that he didn't do. Mayella knew this well enough, and Bob looking stupid and cruel means that she is too, because she is also concocting the same lies. She resents the fact that Atticus exposed them for their lies, and that he made her father look like a fool. She is afraid that if she gets up there on the stand, Atticus will also make her look like a fool.
So, when Mayella mounts the stand, it is with hatred and mistrust in her heart towards Atticus. She is on the defensive, ready to be hurt, offended and angry at anything that Atticus says. So when Atticus refers to her as a "Miss" and a "Ma'am," she interprets it to mean that Atticus is making fun of her, mocking her, and pointint out her poor status to the eyes of the court. She thinks he is being sarcastic, or pulling some sort of trick, like he did on her father, to make her look stupid. She is just so afraid of being exposed, and of saying the wrong thing that she gets confused and baffled at Atticus's kind treatment of her. It's an interesting reaction to have, but, she is on the defensive and highly sensitive, which is why she misinterpreted his intentions in calling her by respected titles. I hope that those thoughts help to clear it up a bit; good luck!
I think mrs-campbell makes very solid points, but I've always seen Mayella's reaction in a slightly different way. I don't know that I'm right, of course, but I've always suspected that because she belongs to a very poor white family who lives just one notch above (and not at all far from) the poor black community, she isn't used to being called "ma'am" and "Miss Mayella." I do know that the terms "ma'am" and "miss" weren't handed out to just every female; they are terms of respect and status. (For example, I believe that for quite some time -- sorry, I don't have dates or hard evidence -- many newspapers would use "Miss" or "Mrs." for white women but not for black women.) As a reader, I've always assumed that Maybella is suspicious of being addressed in a manner that is much more polite than what she is used to.
bc she thinks hes making fun of her