i really need some advice

i have been working out for a few years now, at my school's gym and a gym i belong to outside of school. recently i noticed that after i came home from working out for about an hour (mainly weight lifting), that underside of the head was blue-ish, and the skin right below the head and on the under side of the shaft had a black coloration. after a while it went away. and it always looked the same after i would work out, and it would always disappear.

on Saturday i didn't exorcise as much as i usually do, and when i came home only the skin directly below the head on the underside of the shaft had a black-ish coloration. it has been 5 days since i went to the gym ( and i haven't gone yet since Saturday) and the discoloration is till there.it does not hurt when i urinate, but i am feeling a slight irritation. but what i do know is that it can't be an STD.