I've been dating this guy for 1 month now, and everything is OK till 2 days ago. We went to a birthday party of one of my work colleagues, we got drunk, everything was OK. At a certain point I noticed that he was hugging my girl friends, talking to them... Later, it got to bother my girlfriends and they started to ignore him. I thought maybe he was drunk, maybe he was trying to be nice to everyone.
Now the ungliest point is that the next day, I've been told that my new boyfriend invited one guy from the birthday party to dance, and proposed to give him a ride after the party... now this got me confused.
I am sooo ashamed with my colleagues, for all the situation. The fact that he was hugging my girlfriends and flirted with a man at really confuses me. The man I saw at the paty and the man I see during a normal day are totally different people. I really don't know what to think. Should I openly ask him is he's bi, or... should I pretend nothing happened even if it bothers me?

You definitely need to speak up and tell him what you think and how he embarrassed you with his behavior. Also need to let him know that you won't cope with that type behavior.

Express your concerns about his sexuality, and his drinking. Put up with neither.

Could have just been that he was drinking, a lot of people get the touchy feely vibe when they drink. I don't think just because he invited a guy out means that he is Bi, guys tend to click a lot easier than girls and they become friends a lot easier too, he could have just talked to that guy and thought he was a cool guy and invited him out. But I would definitely be open with him and ask if there are concerns. You don't want to keep wondering.