boyfriend chose another female over me.

Guys. I need major advice. I just recently had surgery and I haven't been able to go anywhere. Me and my boyfriends good friends baby was turning 1 this past weekend so I told him to go since I wasn't able to. There was this girl there and I was told my boyfriend kept hanging around her, he apparently offered her to ride with the store with him, and later her offered to drive her home, all this is happening while I'm home laid up in bed miserable, and mind you he had plans to come take care of me and not stay the whole time at the party. Well he stayed the whole time at the party. With this other girl, he chose her over me. He smoked with her as well, and smoking isn't accepted in our relationship..... my friend told me about all of this, and later I got the truth out of him. He claimed to have no bad intentions but I'm having a hard time believing him.... we have been together for three years and he has never done anything like this before. Please don't say anything rude, this is hard enough for me. I just need good advice from other females.