Are you referring to the brown marking / speckles on the upper shaft of your penis?
If yes, are you an active person, such as doing regular sports or gym workouts, do regular sports training for competitive cycling, or other sport, that kind of thing?
Also, if you live in a hot humid climate, or perhaps even simply wear clothing that encourages regular sweat in the groin region, that could be a potential cause?
If you feel you have been at risk of STD / STI’s then the only way to rule out those is to get tested …