I wanted her, I got her, and now I'm not so sure anymore? Help anyone?

Hey everyone, thanks for reading. I'm a 22 year old grad student and have spent the better part of last semester chasing a 25 grad student. We hung out throughout the semester and had a nice relationship, but we started having sex last week and now she wants to be my girlfriend. I've wanted this all semester, but now that it's happening, I'm not so sure I want her anymore. She's sexy, intelligent, and very kind to me; I have no complaints. I'm just not feeling it as much as I used too...anyone have any advice or personal experience dealing with similar situations? Anything would help.