WOULD YOU TRUST YOUR BOYFRIEND OF FIVE YEARS IF HE DID THIS?

My boyfriend and I have been together since 2006. He started a job in 2008 after talking to a girl at his gym who set him up at a part time mall job that sells high end sunglasses. In 2009 we broke up for a month and after we got back together this girl was texting him about work calling him "babe". I was suspicious but let it go. I think he may have at least taken her out while we were not together but didn't care. We have been together ever since, but on New Years eve he had gotten drunk. His mom kept texting asking where he was so I finally anzwered to let her know he was ok.. just passed out asleep. I noticed there were several texts to this same girl from early that day starting in the morning. The last one said "Nikki. . you're the sh*t! I love you city girl" (I guess because she also works in the Soho store.. and with him in NJ) He did this at 11:55 on New Years while I was hanging out with his friends and then ran down a minute before midnight. She didn't answer him but, to me, he was obviously flirting with a girl that, I think, he had feelings for. Since then my feelings have changed for him. I have lost trust. Am I overreacting? He didn't text ANYONE else on New Years and we both have friends of the opposite sex that I know are really just friends. He apoligized the next day and says he was drunk (but he text her before he got drunk) and when I bring it up he says "get over it". Opinions please!!