I think my bf cheated... Need some advice

So I need some help. I’ve been with my bf for almost 2 years and about a year into our relationship I got tested for STDs just to be safe as I hadn’t gotten tested in a while, last time was a few months before we started dating. Anyways everything came back normal so that was a big relief. Fast forward about 9 months and I get tested again because my gynecologist recommended that I do. Well low and behold I had chlamydia. I’ve been faithful our entire relationship including now so I know I couldn’t have gotten it from anyone else. My question is, what’s the likelihood that I got a false negative when I got tested the first time? Or a false positive when I got tested the second time? Idk what to do. We both took the pills already so can’t retest and I have no proof that he has cheated besides this. How can I be sure he hasn’t? Is there anything I can do? I love this man with all my heart and I don’t think he’d cheat since he knows how it feels from past relationships. And I also don’t want to break up with him with no real proof. I really believe he loves me but this is at the back of my mind everyday. When I found out I had it, I asked him has he ever cheated on me and he denied it of course but what other reason could there be... Ugh idk what to do.

Update: He’s supposedly never been tested so there would be no way of knowing. However, I could’ve been the one to give it to him from someone before we started dating but I ruled that out since I tested negative a year into our relationship.