The first thing that struck me was the timing of her attitude shift. I don't think it unreasonable to think that she had a thing for this guy, and you "stole" him.

I think you should, as Leed said, try to sit her down and talk about this. There has to be a reason, and if there's a reason, it can be fixed. If she won't have that talk with you, THEN it might be time to look at letting her go, sad as that is.

I am sorry. I hope things work out for you.