I get an occasional painful sore on my labia. i have been tested, its not herpes. what could be the cause?

Could be boil: I would say that the second most likely cause would be a boil or small abscess...I would reccomend a good antibacterial soap and see a md with the next lesion for further eval. Could also be oral herpes lesion which is becoming more common.

Hidradenitis: Hidradenitis suppurativa (hi-drad-uh-ni-tis sup-u-ra-ti-va) is a chronic skin inflammation marked by the presence of blackheads and one or more red, tender bumps (lesions). The lesions often enlarge, break open and drain pus. Scarring may result after several recurrences. The lesions can occur in the groin, armpits, and under the breasts.