STD Testing Mix-Up?!

I’m going to try to make this short.

My boyfriend and I started having sex on July 21, 2018. I visited the GYN two days prior with all my test results coming back NEGATIVE. My boyfriend had not gotten tested since May and had sexual partners after.

I got tested through a free-testing, non-profit organization in October 2018, and the test came back POSITIVE for chlamydia. I received treatment. Being that my boyfriend was most definitely, 100% the only person I had sex with, I had to have gotten it from him. He got tested soon after, and it came back NEGATIVE. That caused a strain on our relationship because he thought I slept with someone else.

I told him that his results had to have been a false negative because I got it from somewhere and it wasn’t anyone else.

We had sex a lot after that. Fast forward to this past week. I got tested again, this time at my campus clinic, and my results came back negative. So he was negative.

Where could this have come from? He does not have it, yet I “did.” I am so confused and this is causing so many issues. Input anyone?