Why is a question u need to talk to her and understand y,,, I think she accept kiss..,.,..before but just sex no, (if no to all so not a girlfriend from the beginning)
there are many suggestions one of them that she is not interesting in u, she 've a problem and prefer to let u don't know, or as Rob tell no longer in love with u,,,
To b sure there is the direct way to ask, or the indirect which is to go out with another girl if u find her jealous (e.g. keep looking at u, and being n stress) then there is something made her refuse,,
But if she don't care, then go away and leave her, things are over, and she is just trying to make them in cute way...