Recently I was diagnosed with Chlamydia, but I have only
been with my boyfriend. How can that be?
The fact is you got it from him- no doubt about it-this is assuming you have been faithful to him. Since he gave it you, it only means one thing- he is fooling around with others. I would suggest you speak to him. You both need to be treated for chlamydia.
Then you need to discuss the future about your relationship, because he is being unfaithful. Personally speaking, once a cheat, always a cheat.
