Sexuality check...

If she's the kind of best friend you can tell absolutely everything (every crush, every fear, etc.) to, then talk to her. It might be awkward, but she would help you figure out who you are, and it would be worthwhile in the end.
If, however, you still keep secrets from her, then you do nothing except for wait. You see what happens to your feelings (and don't you dare try to deny them), and in a couple weeks look around and see where you are. If you still like her, talk to her. If not, forget it. (Forget the crush, not the sexuality issue.)
And you are a teenager, remember. There are a lot of teenagers that question their sexuality at one point or another. It could just be a phase, part of your sex driven years. xD Also, it's my firm belief that teenage girls sexualize each other. I think this is part of the road to adulthood, this dark sexuality and confusion.
But always keep in mind that it's possible you are bi. Keep your heart and mind open. That's the best way to figure everything out.
Good luck:)

You are not a lesbian or bisexual. It is normal to feel a little strange or uncomfortable when a girl that you know wears that kind of clothing. If it is only this once, you probably aren't. Your heart may have speed up because you were picturing all of the romantic things that could happen to you if you wore that outfit. Don't stress it.

Ummm... ur a girl, right? cuz, then its not 'gay' its a 'lesbian'
Anyways, don't think ur lesbian only bcuz u think ur bff's Halloween costume is sexy, and the whole "heart sped up" thing is not saying anything.

I think it's not rly helping, but at least I talked about 1 subject^^'

listen. its not that everyone knows everything about sex and sexuality. many a times we have incomplete information. better to talk frankly to councilor.and if you are of other sexuality, then no need to be afraid beb, you have to accept that.
and the best way to check out is to check the frequency of such events. how many times the things like these have happened to you? what feelings you have the most? hetero or bi? talk to yourself clearly.