What's going on between us? What should I do?

15 minutes isn't that long of a delay in reply. Maybe she doesn't want to seem too antsy :] Or she could be busy. Don't read into it so much. It's not a big deal. You just told her you liked her. You both were talking about who you guys liked, more than likely she knew where it was headed and what you were going to say. Give her time to respond.. Don't freak.