gave my boyfriend an std ?!?!

Soooo I was talking to a guy a few months before I started dating my boyfriend . I’m guessing that’s where I got it from . But I had gone to the doctor at the beginning of June and got tested and I had chlamydia . I knew who it was from but by the time I found out, he was long gone and we hadn’t talked in months . I was already with my boyfriend and we were having sex . So of course, I told him and I told him to go get tested and get the appropriate antibiotics cause if we were having sex with no protection, he had it . He went and got tested, and they said he was clean . How is that possible when he was the only guy I was having sex with ? He should have had it too . We were having sex for like an entire month and a half when I had it and didn’t know . We still don’t know how that’s possible . I thought someone had switched either my urine sample at the doctors office or his on accident ... a fluke, something ! Any thoughts ?