J.T.
Hi Kai -
This is very, very, very common in kids this age. If you can visualize human sexual orientation as being on a continuum, where total heterosexuality is on one end and total homosexuality is on the other end, most of us are somewhere in the middle -- and as kids start exploring their own sexuality, they are trying to establish where THEY are on the continuum.
You're doing everything right -- establishing that your love for her is not contingent on her orientation, talking to her openly about sex (don't you wish OUR parents did that? ;) Also, it is now 'cooler' or 'more okay' for girls to be more 'tomboy', into sports, smarter in school, etc. This can sometimes make a girl look less than a girl according to our social standards. But, that doesn't mean that her orientation will necessarily change.
I went through this same thing with my son when he was 14 (he's a gentle, smart kid -- not your typical 'male'!). Six months later, he had a girlfriend. Who knows, he may end up changing his mind again, but it is all part of the process of figuring out who he is.
As to worrying about your daughter's exploration -- I had this conversation, too -- it went something like 'I know that there is a strong feeling to experiment and try this out, but experimenting can cause all sorts of weirdness -- parents get upset, friendships can be lost, and sometimes it can even end up in court -- you're simply too young to be engaging in partner sex, and for now, experimentation is something you need to do alone.' It wasn't an easy conversation, but once we had it, we could talk about it again. Now I think that I will actually be consulted when he is ready to have partner sex!
Anyway, hope this helps -- deep breath, Kai -- it's all normal!
J.