Hello,
Welcome to ehealthforum and thanks for writing
Sore on the labia can be due to:
1)Infected hair follicles - can form small abscesses, which can cause a small swelling, and they are associated with redness and throbbing pain.
2) Bartholin's Cyst - Is located midway of the labia, and protrudes into the vagina and causes discomfort.If there is any infection then abscess is formed.
It requires a direct physical examination to rule out the exact cause.You can consider applying Neosporin ointment twice a day for.If it does not subside then consider visiting a Gynaecologist for evaluation.
Take care!