C.O.
My sister and her daughter had this same problem. It ended up being a boy bullying her at school. There is something that is going wrong with her but she is just not able to express it properly. My sister had to ask her daughter every day for a least a week before she told her the truth. My niece told the teacher repeatedly and the teacher did nothing. So my niece just thought that no one would help her. My suggestion is to keep asking questions as to why she might be behaving this way.