I had a chick who did this to me a while ago, We were in a relationship for 3 years, went are separate ways. Then out the blue she came back into my life for another 2 years, then left again.
During that time I had not experienced a women coming back into my life so I watched what was going on as a learning experience. Keep in mind that I had move on in my life so i had no emotional commitment to her, i was using the relationship as a study and for the available sex.
Afterwords I realized what was going on : I was a comfortable place holder for her, during the time she ended a previous relationship and as she was searching for the next relationship.
That's it. So i wasn't complaining about the sex, but for me the amount of time i was spending with this chick it wasn't worth "just sex". I like women that add value to my life, not just hanging around like leeches.
Bottom line: Never press the rewind button. when the relationship ends, it has ended. (In her mind)
What it sounded like for you post is that she was going to use you as the place holder, but found a better offer.
There was a reason your relationship ended before, It may have not even have been because of you.
If that is the case then don't put too much stock in the situation because she is going to probably reject you at first. Now, if you love and want to be with her then it is worth the fight even if you lose.