Maybe screwing girls in the backseat of your car isn't such a bright idea after all? Especially if you aren't even in a relationship?
Anyway, do you actually want to go out with her? If not, why do you care? You used her for sex, and if you don't want to go out with her, what further use do you have? If you do want to try to have a serious relationship - well, maybe you shouldn't have slept with her before you figured that out. If the latter is the case, maybe you should apologize for trying to get her to have sex with you when you both weren't clear on exactly what kind of relationship you two were in.
Oh, and she's only partly mad at you - I'd say she's probably just as irritated with herself, and with her uncertainty as to what is really going on, as she is with you.