How has this happened

I've been experiencing a lot of pelvic pains. I went to the doctor who put me on antibiotics in case of infection. He also asked me questions about how long me and my partner have been together. When we googled the antibiotic they are given to people with chlamydia. Me and my partner both got check soon after getting together and we both came back fine. I haven't been with anyone else and he swears he hasn't and i believe him. Can you get chlamydia or similar infections without cheating?