I'm 21 y/o, and have had this problem for probably a year maybe two. It doesn't always happen and can go some periods without it. Basically what happens is that after I masterbate or have an erection for an extended period there are times when I pee that cause a sort of pain on the middle outer part of my urethra. My personal speculation is that it is due to my penis growth in some way, as when non erect it sits at maybe an in, but the largest erection ive gotten has been just about 6 inches. So I figured it couldn't hurt to ask about it.