I am confused and angry... What should I do?

A few weeks ago, I met my best friend's neighbor for the first time and he is very good friends with her. We fell for each other almost immediately and did some things in his room that night which I regret... Then I stopped talking to him for a while but recently we have started talking again and have hung out a few times. Well, this isn't the problem. The problem is my best friend. She has been acting differently and strange and has kind of stopped talking to me since, or just giving me b*tchy replies. She tried to tell me that he told her he liked her the day after I first met him. But I asked him and he said that never happened and he used to like her but not after he met me. He tells me she isn't talking to me because she has a lot of messed up things going on in her life right now but I found out last night that she spent the night at his house and cried to him about it all... she doesn't even want to talk to me, her best friend. So should I be jealous.. angry.. confused? I don't know what to think about this. She had always encouraged me to get with her neighbor and said she wanted me to be with him so I doubt she likes him... But I'm not sure anymore..