Can't tell if my boyfriend is cheating?

Hi, all.
I’ve recently gotten into a new relationship and he’s a very nice guy, we talk a lot, we’re very comfortable with each other, everything seems fine. But, he has a friend, whose a girl, who lives a few hours away, and when he goes out of town, he’ll stay at her place. I’ve read a few of their messages between each other, and from what I see, she loves to flirt with him. He’s told me, she is aware of who I am, and everything is fine and if I needed to, I can read his emails. But, I’m not sure how to bring this up, how do I ask? I’ll tell him, what you’re doing is hurting my feelings and he tells me everything is fine, he’s just looking for work out in the city and I’m overreacting. Am I overreacting? Sometimes, I feel like he’s hiding things from me and I don’t want to start a big argument.