What Causes A 1/3" Medium Brown Spot On Penis?

Hello, I am 19, about 6'1" and 195 lbs. I am not a virgin but have not had sex in a while and am about 99% sure I do not have any stds. However, I woke up this morning to find about a 1/3" medium brown spot on my penis, right near the base of the shaft. No pain, itching, irritation, no change in skin texture. Flat surface. Not completely round. I am incredibly curious as to what this could be. Any information I can get on this would be greatly appreciated.