For the past couple of weeks (probably at least 3 or 4, maybe more) I've noticed that my penis seems to have a discoloration near the base of the shaft. Actually it looks like I have a tan line about halfway down the shaft. The base looks tanner (darker) than the other half. In fact, the top half looks normal while the bottom half looks excessively dark. I know that it is not a real tan mark because I have never tanned my genitalia (and definately not only half). Can anyone explain what could be causing this and what I should do?