I know I'm generalising a bit here:

the thing with women is, their feelings for someone come and go a lot faster than it is for men, i.e. they may initially be attracted, but if the guy does not do the right things to stoke the flames then those feelings eventually die out.

You can see this at bars and clubs: as guys are generally attracted to someone based on looks, the girls they have the hots for will not change from the beginning of the night, to the end (Beer Goggles notwithstanding), whereas for girls, who are more-so attracted to a guy based on what's on the inside (a darn good thing given most men don't look like Ricky Martin), they will gain and lose attraction for different guys throughout the night, depending on how she sees them through e.g. their mannerisms and body language.

essentially, just because a girl may have been interested in the beginning, unless you fan those flames and keep it burning, her interest will fade over time.

And I agree with those who suggest you take a break from her, hard as it may sound..