Hmm, she knew you were a virgin, and still slept with you knowing she was seeing another guy? In my opinion, and maybe I'm old fashioned, but that wasn't cool. Your first time is supposed to be special, she probably should have told you this was just a casual thing before anything happened. Not judging anyone, but that wasn't a kind thing for her to do. Your first time is a pretty big deal, and it sucks that it ended like this.