Boyfriend a cheater???

So I recently had my blood tested for a full STD screening. I'm 18, been sexually actively since I was 14. I've never slept around nor have I ever tested positive for any kind of issues down there. I have annual exams like every other girl on here. I've been dating my current boyfriend for about 8 months now. I got normally tested at the gyno for STD/infections and everything was negative after our first month of dating. We've had our ups and downs but I never thought he would cheat on me. That's why I was so surprised when I recently got tested and was positive for chlamydia. This was super scary and gross to me and took weeks to mentally get over the diagnosis, even though I was treated and fully recovered 5 days later. Is there ANY OTHER WAY I could've contracted this??? I don't cheat!!!!! I just want to know if there's a possibility that he didn't sleep with or receive head from someone else.