AIO - Does Snapchat glitch?

So my boyfriend and I have had some minor issues with an ex of his. After an instance where I saw messages off her that were inappropriate I asked him to stop messaging her. He said he would, but apparently when he told her this she blew up and she said he should have waited to be with her and not got into a relationship with me. Then, a few months later I saw a notification pop up from her on his phone. In his messages he had one meme she’d sent which he’d reacted to, then another meme which was the message I saw. I asked and he said he’d forgotten to mention it. They then bumped into each other when he was working at an event where she apologised and asked if they could be friends. I wasn’t comfortable with it because of the above and asked that he didn’t do this. Anyway, I had to go into hospital for surgery yesterday and stay in hospital overnight. I grabbed his phone and clicked on the three lines (Android phone) to flick between spotify and maps and I saw snapchat was about 3 apps back as you flick across. I don’t have an Android, but this means it was open fairly recently, doesn’t it? Then I went into snapchat to see her name and ‘opened one day ago’ underneath it. He flat out says he hasn’t messaged her. Can snapchat glitch and show this from a message a while back? I feel like little lies are where things start, my dad was a serial cheat and I’m desperately scared of letting this go and him lying to me further. He offered to put an app on his phone so I could track his convos, but I don’t want to do that. He’s uninstalled Snapchat now so I can’t see any more detail. If I could get him to reinstall it, what would I look for to see if they had been talking recently? Should I leave now or am I massively overreacting? We just moved in together and even if he is lying, it’s just a message, right?