The Ex is back (Advice please) going crazy

TIa
My husband and I have a great trusting relationship but when we first started dating he had a few girl friends that he hung out with and texted, one was his ex. I didn't think anything of it. It had been years since they had been together. On his 21st birthday he passed out at my place and I wanted to text his mom know he was safe, so I went through his phone and saw their text convo. They talked about their old sex story's, made fun of me for one of my favorite movies. Nothing physically ever happened but to me this is cheating. The next day we didn't even fight I bawled and told him i can't be with a cheater. He apologized a 100 times and deleted her from facebook and his contacts and for some reason (thankfully...I think) I believed him and gave him another chance. In the 5 years since he's never given me any reason at all to not trust him. Well last night I grabbed his phone thinking it was mine, facebook was up and I seen she sent a friends request, Idk when and he hasn't accepted and he never denies them. It was just a slap in the face I guess he hurt me so bad with her, I'm worried they're messaging back in forth again. I don't want to go through his phone but I can't get it off my mind. This is why I knew I couldn't handle being with someone that has cheated. In the back of my mind I've always wondered if she's the one that got away and I'm the one he settled for I just want this doubt to go away but I can't ask him if he's been talking to her.