Did he cheat?

Ibreus
My body started to show symptoms that something was wrong my vagina I went to my GYN I was diagnosed with chlamydia honestly I never cheated on my boyfriend and when I told him he was the one who cheated he swore he never did something like that he was very confused even upset because I accused him. I broke up with him but then I don't know why I feel like I did wrong I m kinna confused I don't know whether he really cheated or maybe he had it before we got in a relationship. We've been together for a year now and he never gave me any reason to doubt him except this result from the Doctor.