Be careful with this chick bro. My last relationship started almost exactly the same as what you're saying. "Friends" that hung out every chance we had, eventually felt each other up during a movie, started fucking all the time after that, and never dated. She didn't want a relationship with me for w/e reason. Eventually we got together officially, and lasted for close to 3 years but...turned out she was fucking one of our co-workers.

Oh, and the sex was very good there too, but it sucked at first. I think I turned her into a whore because she was really shy and timid, and boring in bed at first...and eventually she turned into a very "outgoing" fuckmaster that would try to convince me to do naughty things to her in public....moral of this post is that good sex = evil girl.