What Causes Rashes On The Butt, Hips And Waist Line?

Hi, after i had oral sex with a guy i got a rash on my butt, hips and the back of my waistline. after a week it started to dry up but within that week the same guy and i had unprotected sex and i got a few more bumps and about two to three weeks after sexual intercourse my throat seems to be slightly sore. the rash started as pinkish buttons and a few of them were filled with puss but when they dry they are leaving black spots. the fresh set of rash seemed to appear in my pores but accompanied with the same type of buttons i described before, so my pores on my waistline and underarm is slightly raised. i need your opinion on this matter please.