What Is The Recurrent Painful Red Blister In My Labia Majora?

Hey I have this red blister looking sore on my labia majora. It comes and goes.Its very uncomfortable and painful. I hate for my underwear to rub up against it. I have recently been tested for genital herpes. I had the same bump on by inner buttocks. The doctor did a culture on my old blister and a blood test. Both labs came back negative. They also tested me for other STD s and they also came back negative. I do shave alot and sometimes not as careful as I should. What do you think this blister could be ? Do you think that the labs could have been wrong?