Blister like spot on shaft of penis?

Patient: I have a blister like spot on my penis, halfway down the shaft, its uncomfortable. it has been there for a few days, it started off like a spot, i scratched it and it became what resembles a blister, with a greeny white liquid inside, when i pulled the skin off it bled. i have been having daily sex with my girlfriend using a condom and once without but didnt finish. my girlfriend has no stds that she knows of. what could this be? should i avoid sex and do i need to treat it?

Symptoms: Pain, irritation