I think she THINKS she loves you. My ex (on and off gf cos I cant stand her crying) was the same. We had great sex, multiple orgasms (for her at least lol) and so she equates that with love. She actually said she loves me the first time we had sex, after the seventh orgasm or so. She had said it prior to that though...
Anyway you have to tell her. The girl if she is young probably doesn't love you and really loves the sex but can't realise it due to her lack of maturity/brains...course if she's a bit older like 19-20s it could be somethin else.