What's Up with my best guy friend?

My best guy friend and I have been super close for a while. We can talk about anything and we know each other so well its like were related. Though, some people mistake us as a couple, and they always accuse us flirting. Lately he's been acting "distant". When it's just us we're our normal selves, but when his guy friends some around (my friends too) he ignores me, nudges me, and makes witty jokes about me. HELP