Sometimes a small particle (a la talcum powder) can imbed itself. This happened once to me. Is it a bump, or does it definitely have a head (like a pimple?)
Keep a close eye on it and don't let it go too long.
As previously reported, depending on what the "pimple" is, could be late signs of syphilis or beginning of herpes... all have similar signs, but how the spot appears is key. Don't be stupid. When in doubt, CHECK it OUT!