From your symptoms,it looks like a boil. A boil, also referred to as a skin abscess, is a localized infection deep in the skin. A boil generally starts as a reddened, tender area. Over time, the area becomes firm and hard. Eventually, the center of the abscess softens and becomes filled with pus. Finally, the pus "forms a head," which can be surgically opened or spontaneously drain out through the surface of the skin.But it is also important to rule out herpes completely. It is spread through direct skin-to-skin contact during oral, genital, or anal sex with an infected partner.
The only way of ruling it out is by getting yourself tested for genital herpes. Laboratory tests include: culture of the virus, direct fluorescent antibody (DFA) studies to detect virus, skin biopsy, and polymerase chain reaction (PCR) to test for presence of viral DNA. I sincerely advise you to consult a dermatologist and get it ruled out.
Hope it helps.Take care and pls do keep me posted on how you are doing or if you have any additional doubts.