huge pimple or boil on penis

First I would treat it with hydrogen peroxide, it should bubble up when you first apply it, but it will clean and dis-infect it.

If you have some hydrocortizone cream,you might apply a little,then wrap it with a small amount if gauze. If you can cover all the way around,use a piece if medical tape to hold it in place. If your a guy who grabs his crotch alot, try not to so it won't get more irratated If after a day, it doesn't look like it's getting better or staying the same, you should  have it looked at by a Dr, I have never heard of an STD with that type of sympton. But don't let it go untreated!! I poked my leg with the end of a file, small puncture wound, by the time I got to a Dr. it was infected, had to go in for 7 days for hydro therapy, and ended up with a 2.5" scar, which is not real attractive.