Hi, I'm a 28 years old man. about three months ago, after a strong masturbation in which my foreskin didn't sem to retract properly, a black spot appeared on my penis. It's dark and pretty long (It starts on my foreskin and it' reaches the end of it), and it's on the part I usually masturbate. The strange thing is, it practically disappear when my penis is erect or if I stretch it when it's not erected. It doesn't hurt or anything like, ithe skin is just a little rought in that part. What could it possibly be?