While it is of course okay to question your sexuality, and it could be possible you are bisexual, I agree with the above posters that before worrying too much about whether you're bisexual or not you might explore the possibility that you may simply be jealous of the fact that your friend has someone new in his life that he's spending a lot of time with. It would be completely understandable, as it disrupts what used to be your usual dynamic. Before jumping to conclusions I would suggest just giving it time, both to see if you develop attraction or feelings to any other males, and for the routine to settle again with your friend, his girlfriend, and you.