Fiancé gave me chlamydia

I’ve been with my fiancé for 5 years. I was feeling sick not myself and went to the doctor to get tested, came back that I had Chlamydia!

Talked to my fiancé because I haven’t been with anybody else but him

He swears up and down that he hasn’t been with anyone!

He thinks by something else or the doctor called the wrong person! He is making me doubt myself, I don’t know what to do I know I should believe him but now I’m confused