Am I overreacting?

My boyfriend went swimming at his female ms friends house tonight. Didn't tell me before or anything. Well he waited around for me to get my break at work. I asked him where his swim trunks were and he told me he went swimming in his underwear. I got really upset about it because he had thought to grab other clothes but nothing to swim in. He kept telling me it was a last minute thing. I told him it wasn't if he had grabbed other clothes. I've hung out with her before but I still don't think that it was okay that he was in his underwear around another girl without me there. Am I blowing this way out of proportion or not? We've been dating for 8 months