chlamydia

I got the call from my gynecologist after my annual exam saying I tested positive for chlamydia. My boyfriend and I got our antibiotics, but there's just a lot of confusion about where this came from and how it happened. Both our suspicions lie with my shitty ex likely having been seeing other people behind my back, since I was last tested only earlier this year and was negative for everything. 
The only thing I'm the most upset about with this diagnosis is that my ex and I never unprotected sex. My boyfriend asked all his previous partners and they all said they had tested negative for everything as of recent. I know that chlamydia doesn't just happen, and I really trust my boyfriend hasn't been cheating on me...is chlamydia like any other STD that can be transferred via skin? I know it sounds silly and I will be getting comments calling my boyfriends fidelity into question....it just kinda blindsided the two of us because we've only ever really had unprotected sex with each other.