I'm sorry you have to go through this! I know it may seem scary to bring your child to a therapist, potentially drudging up topics she may not want to talk about, but it seems like that may be one of your only options left. But not to worry- therapy may seem more scary for you than her.
I'd call the pediatrician and see what therapist they reccommend, or if they could provide some insight on a potential medical problem with your child. Consider having a private conversation with the therapist first and explaining the situation. They may question you about the kinds of magazines you have around the house, books, etc., and then about the relationships your daughter has with people that she saw surrounding the birthday in question. Do your best not to feel accused or uncomfortable, they're trying to do their jobs and help you.