How would you feel?

Okay so like 5 years ago In highschool I had a bf who cheated on me. We were doing long distance (I had to move to a different state) and he called me and told me. He said the girl skipped school with him and gave him head. He promises that its all that happened and the girl swears too. Well regardless it was 5 years ago, we got back together a year or so after we graduated and back and better. ANYWAYS, my brother came over last night and Stayed the night (he’s 15) and he was talking to us about this new girl he’s been talking to. He told us he snuck her over one night out parents went out and she gave him a bj. Later that night my man was like “I bet you he went all the way and just lied about the bj so he didn’t feel guilty” and my heart sunk. It just made me wonder if thats what he did. I know if I brought it up I’d sound crazy seeing it was 5 years ago, but like what would make him say that? Why would he believe my brother lied? Would you wonder like me? Or am I over reacting