I’ve had this situation before. My boyfriend from years ago messaging someone in bed next to me the cheek of it. Saw ‘Emma’ flash up on the screen, and I asked him casually who he was messaging and he gave me his lad mates name. I was very young and naive.
Hidden messages all night from me, I wept myself to sleep.
Confronted him the next morning and he said I was hallucinating and told me to go to the doctors. Then ‘broke up’ with me for being ‘mental’ and ‘controlling’. I apologised and begged for him back.
Cue 6 years of being in a horrible relationship feeling torturously anxious daily with an abusive, lying, cheating arsehole. He admitted it in a fight years later, and a long string of other girls too.
We are sensitive creatures. If you have niggles about it then trust in it.
I’d confront him and ask him to show you the message. If he’s got nothing to hide then he will oblige.
If he already or accidental ‘deleted it’ or makes up another excuse then he’s a cheat and you deserve better. Don’t torture yourself by dwelling on it, find out as quick as you can, be straight forward and blunt because by delaying, overthinking and playing detecting will only punish you and no one else.
I really do feel for you it’s a horrible feeling. Good luck 