What is this girl confused about?

So this girl and I have been getting to know each other quiet well for about a month now. We've caught up a few times at parties and the like. At one of the parties we ended up holding hands and getting pretty close. She told all her friends she liked me. Last weekend we went to a formal type thing together, except each with different dates (its just the way it worked out , we're all good friends and her date knew I liked her so there was no funny buss. going on). We hit it off there, letting me get really close holding her and stuff. Then we went to an after party. In the space of about 20 minutes everything changed. She appeared to lose all affection towards me. Since then (about 4 days ago) our conversation have been painfully slow and shes not longer making an effort. What could possible have happened in that brief period of time to make her change her mind so drastically. i cant think of anything that I might've done and she lead alot of the intimacy at the event. All I can think of is that she might've been disheartened by me not making a move on the night. Help me out guys, I'd really appreciate it.