You've not really seen yourself as gay or fully recognised yourself as gay for very long so you may not be gay, you could be bisexual or you could be gay but you could just have one off feelings for Shalia.
It's been said and I agree with that no one is 100% one sexuality or it's rare to find someone, basically most people you come across have tendencies and only some people choose to act upon them or meeting the right person could make them act upon the way they feel.
I don't think there is any real rush to put a label upon yourself, you're 15 so you're still young and hormones are flying all over the place, I'm not going to give you the 'Are you sure you're gay' crap because I've known since I was 13 that I was bisexual and it's stayed the same.
Sexuality is fluid so if to say you were bisexual you may be primarily attracted to guys but are attracted to the occassional girl.
So, calm down and there's not a real rush, take your time and I'm sure in time to come you'll be able to find a label you're 100% comfortable with, or you may decide not to label yourself as anything which is absolutely fine too.