For several months, upon getting an erection, I have felt a sharp, stinging pain, usually right at the front of my urethra. The pain is much worse when I've had sex several times in a week, and usually builds up until I have to stop having sex or masturbating completely. After waiting around a week it is usually manageable, but builds back up again quickly. It is particularly uncomfortable when the front of my penis rubs against something, usually the inside of my pants.
I am almost certain, though, that this is not an STD, I only have a single partner, and neither of us have ever had an STD (she has had a UTI). I have also visited my physician about this issue, and testing shows that it is not an STD, but I cannot find the source of this problem.