What Causes Dark Spots On The Penis Head?

Hi, my name is Derek. I am 18 years old, I am 5'11, I weigh 135 pounds. Yesterday, at around 6 PM, I found 3 or 4 very small black dots on the head of my penis. They don't itch, they don't hurt, they're not irritated, they're not bumpy. They just look like small beauty marks. The strange part is that I didn't have them in the morning and then by the end of the day, they must've just suddenly developed. Do I have an STD or is it just something natural or minor? Please help.