It's got nothing to do with her feelings for you changing; it's that she is either

a) less over the breakup than she's letting on (ie, not ready for another relationship), or

b) not in the position she thought she would be in when she gave you that hypothetical situation. It's easy to say things like that (and mean them) when it seems like a far off possibility. But then, when the time comes, you realize you could never have followed through in the first place.

Don't take it personally, and give it some time. She might come around. And if not, this really isn't worth ruining a friendship over.