What causes red spots on your penis?
Where has it been and what has it been doing? Just one or more ? Could be a bite, friction burn from masturbating. It is probably not an STD if it is only one. Pimple, yes, you can get one on the penis.
These are very common. It may be a very mild STI or just a very small case of Herpes. Don't worry, just go see your GP.