[25/F] / [25/M] Losing it over my partner's past situation - called him at 1 AM last night.

I [25F] and my partner [25M] have been in a relationship for two years now. (I hope this remains a judgment-free space.) I’m not usually one to share my relationship problems with friends or family. However, last night, I felt compelled to check his phone — a move I'm typically against. I was anxious the entire time, but curiosity got the better of me. I discovered recent exchanges between him and another person, discussing sports and occasionally flirting. As I scrolled up, I stumbled upon a particularly unsettling conversation from November of last year, where they met up with a mutual friend after his work. He had told me he was out with the guys that day. Honestly, until now, I didn't question his story; I fully trusted him. I haven't been able to sleep since discovering this. So, I'm reaching out for advice: should I confront him about what I found and express my concerns, or should I try to brush it off and convince myself it’s not a big deal? (For context: they never truly "dated," just had fun together, and he wasn't interested in a serious commitment when she wanted one. He and I were friends for a long time before we started dating, and I was aware of their casual situation.)