I think she's confused herself. She seems to like you... Unless you're the only guy in the whole world that she could've asked to prom (just kidding).
Keep on talking to her, make her laugh and everything that you do... Don't bother if you have to talk to her first. Initiate! girls like that! Be friendly and talk and listen to her. Treat her well. It'll become obvious to her that you like her (if you really do).
Maybe you did something or said something that bothered her at prom. OR maybe she was expecting something more from you. If you want to find out, just ask her casually, "does anything bother you?". You can even ask her out - you can do something that you both like.
There you go.... Enjoy and take care!
