STIs - Similar Experiences?

I've been experiencing a lot of abdominal pain lately and my doctor asked me to do a test for chlamydia. Despite the obvious (unprotected sex) , I can't think of how I would've caught it. My boyfriend is the only guy I've had unprotected sex with, I'm the only one he's had unprotected sex with and we've both only slept with two people.  
I'd appreciate it if I didn't receive any lectures about "being stupid and not using a condom". I'm perfectly aware of the risks I take with my own sexual health. I just wanted to know if there was anyone out there with a similar experience?