Most Helpful Girl
You say you dated this girl for a short time? There's probably a reason why you're not still together, and if you look the same now as you did then, I'd hazard a guess that physical attraction isn't the important factor here.
I would imagine that she discovered something about you in the time while you were dating that made her reconsider whether she actually wanted a romantic relationship with you. I have a number of friends that I think are pretty darn attractive on a physical level, but I would never seriously consider dating them because of various incompatibilities in our personalities or values.
Granted, if she actually does feel this way, it's pretty misleading of her to tell you that she thinks you're attractive and to otherwise flirt with you.
Also, since you seem to be pretty convinced that she actually IS into you, you might be interested to know that in friendships between men and women, it happens pretty frequently that the male half has your mindset while the female half feels more platonic. This has been my experience in talking among my friends, and a study to this effect was published in the Journal of Social and Personal Relationships last year ( link Seems like kind of a "well, duh" conclusion to me, but maybe not to others.