What Causes A Zit Like Development On Shoulder Leading To Pus Discharge?

i have something on my shoulder witch i thought was a body zit or something. but puss just keeped comeing out, in tell i had to stop because it was hurting so much. so ever so often i go back into the bath room and wash my hands before and after i get a good amount off puss out and and put some first add stuff on it. its red at least an inch around it and like blue around the hole where im getting the puss out of. what should i do??