Is this cheating?/Should I trust him?

I’m never one to snoop but my boyfriend left his Instagram logged in on my phone and I noticed he got a message and we’re both very open we never hide anything like I go on his phone and he goes on mine, but I saw this. Is this considered cheating? I know he didn’t and would never actually meet up while we were dating but like wtf. I asked him about it and he says it’s a glitch because he had his friends account logged into his Instagram too a while ago and it messed with his dm’s so that girl was just a glitch from his friends messages too. Then I went to her profile and he follows her, watched her story (of that day), and liked at least 7 pics and normally I wouldn’t care at all but I care because the photos are all nude and sexy and the captions are like looking for a boyfriend and sex is natural and sex is always the answer and blah blah blah, like should I trust him that it’s a glitch?

He’s never cheated or anything close to it before he’s very loyal and I used to trust him 100% but now I’m not sure. Idk what to do ugh