Worried my daughter was messed with?

My 11 year old daughter came home from a friend's house yesterday, very quiet and withdrawn. As she is usually happy and very chatty, this worried me. I asked her if she was feeling okay and she said yes, she was fine. I have worried about her friend's father before, but figured it was just because I was d as a kid and i'm afraid it will happen to her, too. She has been really quiet and has been spending a lot of time in the bathroom lately. Usually she tells me everything and now I can't even get her to smile. I'm totally baffled-- help? I don't want to overreact but I need to know my baby is okay. What could it be?