I need some advice

So me and my fiancée have been together nearly 3 years and basically he gave up smoking 9 months ago just before we found out I was pregnant and also we both used to smoke weed together and we agreed that we would only do that together and then obviously we agreed not to do it again because we were going to have a baby. In February he went to visit his friends for the day who live an hour away and I know what they are like so I made him promise me that he wouldn’t smoke or do any drugs and he was so genuine and swore he would never. So he went and it was all fine and when he came back I asked if he smoke or did anything and he said his friends were but he didn’t.

So a couple of months ago he was trying to show me a picture of something from his friends group chat and he had to scroll up to find it and he clicked on a picture of himself but he closed it so quickly I couldn’t see and I wasn’t even sure he was in the picture but because of how quick he closed it I had such a bad feeling in my gut. For these past couple of month I’ve had some opportunities to look at his phone but I’ve tried to trust him and I ignored it bc I felt weird doing that but just now he’s asleep and his phone is with me and I couldn’t help but see what that picture was finally. And I’m absolutely heartbroken because it’s just what I thought😔 it’s a picture of him at his friends place rolling a joint. I know I shouldn’t have looked but I had such a horrible persistent feeling for these motnhs. I’m so upset because he promised so genuinely that he wouldn’t and didn’t do anything and I can’t believe how he could just lie to me like that it. I was so upset I started crying and shaking because he knew that was my main concern about him going to visit his friends and at the times I was 8 months pregnant and I didntwant him to leave me but he still did to go and smoke. Even before I saw the photo I was like to him “ how do I know you didn’t smoke when you were with your friends” and he would be so genuine and be like no no I promise you I didn’t. I’m angry disappointed and heartbroken. I really can’t keep this to myself I want to confront him even if it means admitting I looked at his phone. Should I??

It may not seem like a big deal idk but to me it is because he lied and he knew how upset I would get and because he quit smoking and I’m so upset he did it again especially when I was so pregnant. And not just smoking but weed ! When we said we would only do that together and then we said we wouldn’t do it again because we are parents