What Causes A Red Sore On The Labia Majora?

I have a red sore on the lower part of m labia majora. It is sensitive and hurts when I urinate or bear down to have a bowel movement. In the latter case, it feels as if the skin is being stretched. It is reddish, but does not bleed. You can only see it if you move the skin above the perineum. It sort of looks like a canker sore. It is painful. What could it be? I have been tested for all STDs in the past, as my ex hsband cheated and came back negative for everything.