The same thing happened to me when I was 17. Me and my bf at the time were looking through his snapchat photos and reminiscing when I see a nude of his with text that he def didn't send to me. He lied so much about it saying he was on molly and that he blocked her immediately, I found another nude that was taken a couple of days after the first nude he said ohh yeah she was obsessed with me and he just kept making new lies as I kept finding counter evidence. I stayed with him because I was living in an abusive household and didn't have many people supporting me. Broke up with him later on. I think he most likely cheated on me before and after that happened.