It could be that relationships with guys get complicated and another girls is less threatening. Are you a virgin girl?
I've been in that kind of situation, and I've been described as "sequentially bisexual" after I completed a long test a few years back.......there are times when I end men much more sexually attractive than women, times when I'm far more attracted to women than men and times when I find both pretty much equally sexually enticing.
You may be coming to the realization that you are a lesbian, or you may be bisexual in a time when you find women more attractive.........either way, wondering why does little. Enjoy your attractions.
Because sexuality is fluid, then again youre young so it'll happen more and more..
Its also called being bisexual..nothing saying you had to like either boys or girls exclusively