I think my fiancé cheated with ex wife

I'm at a loss. A few months ago my fiancés ex wife claimed he had cheated on me with her. She sent me screenshots of him taking selfies and a dick pic. When my fiancé got word he deleted his texts from her but maintained he didn't cheat. They separated last March and we started dating last May and he's always said he hadn't slept with her since before they separated. Well he now admits he slept with her when we first started dating but a week before we were "official". So he says it's not cheating but that means he's lied for months about sleeping with her when he knew me. I can't help but wonder what else he is lying about. It's never set right with me. Especially when he deleted all the texts. He is good to me and I know he loves me. But I can't help but feel like he's lying and I just don't know what to do!!!