What the fuck

Hi everyone. Monday I went for my annual checkup at the gyno and it was my first time at a new office so they said they were going to run an Std test. I told them it wasn’t necessary as I am in a 5 year relationship and got tested at the beginning of it and we’re committed to each other. She told me it was strictly standard procedure for new patients not to think twice about it. I didn’t until Wednesday morning I woke up to a positive chlamydia test. Instantly my heart broke and I obviously panicked and called my fiancé. He swore up and down on everything he did nothing and I knew I did nothing so we were like wtf. We came to the conclusion it must’ve come from his ex girlfriend who cheated on him in a previous relationship. We thoroughly believed this until his test came back negative. WHAT THE FUCK !!!!! How the fuck could my test be positive and his be negative???! We have sex multiple times a weekend and I have honest to god never done a single thing I would NEVER. Is there anything that can throw off a test? Medicine that could mess with the results or anything like that ??? He believes me that I did nothing but I can see the sadness in his eyes and I hate it because I know I did nothing. Is it possible he could have had it so long that it wouldn’t show up as positive? I literally can not understand how this could happen and I’m sick to my stomach because I know how this looks on me. Please help