overly flirty boyfriend...

So I did a bad thing and checked my boyfriend messages. I know it's bad I just felt an impulse and I saw some messages that made me uncomfortable. They were overly friendly with someone I don't even know. She was talking about going out with her friends and he said "but you could come see me instead". I'm really upset considering we've just been through a similar situation with another girl that he now promises he never speaks to. If I bring it up he'll say that it's just innocent and not flirty but it's making me really uncomfortable and upset. Any advice?