How to know if a girl is playing you or just awkward?

Long story short, went out with this girl like 5 or 6 times. She really acted like she liked me. We made out and then had sex. BTW she knew I was a virgin so I'm sure didn't expect great sex. But like a week after she seemed all weird and even said we were just friends. Could she have been just looking for a lay, juggling multiple guys, or just awkward after we had sex?