From what I noticed just being that age, and around girls that age, is that most seem to go through a disillusionment faze. Do you know what you actually did? or is it just that she thinks everythings suddenly wrong?
Best you could probably do is ease into the topic of the problem. Jumping at people and demanding answers only puts them on the defensive. Y'Gotta just talk about stuff, laugh, build a safe relaxed talking enviroment and she might let you know whats on her mind.
14/09/2007
The spinny chair of doom!, Queensland.