Boyfriend trouble?

My boyfriend asked me if in the 11months that I have been with him if I ever cheated and I haven't but the reason he asked was because his started burning when he urinates 3 days ago. He says it was pretty strong the first day and know it has diminished. I don't have any symptoms of any STD. I am also sure he has been faithful 100%. What can he have? If we are in a monogamous relationship can we bring an STD to the relationship?