How come every time i have sex with this certain guy i get a big painful bump filled with puss in my bikini area , and i didn't shave so it's not that?

See a doctor: You should have this looked it to see if it is a skin infection or other infection that needs to be treated.

Need a visit: If you are having recurrent drainage of pus from a vulvar area, then this might be a sign of a vulvar abscess. This should be evaluated in person with your ob/gyn. If not treated, it could become a surgical problem. Go get checked out and good luck!