Why is she being so cold? I've done nothing wrong?

She's immature. It's a game that some immature girls play wity guys. They get guys interested in them, then they act like this toward the guy hoping that he will go away. Then on to the next guy (which is what you're seeing happening now). In a few weeks, she will treat him like she's treating you now. I would just ignore her and give her back her own medicine.