Thanks for posting your query.
For this, there are few causes and most common is Bartholin's Cyst which Is located midway of the labia, and protrudes into the vagina and may cause discomfort during sexual intercourse.
Bartholin's cyst is formed when a Bartholin's gland is blocked, causing a fluid-filled cyst
to develop. Bartholin's cyst is not an infection, although it can be caused by an infection, inflammation, or physical blockage (mucus or other impediment) to the Bartholin's ducts (tubes which lead from the glands to the vulva
Second, Infected hair follicle may form small absess, which can cause a small swelling, but it is associated with redness
and throbbing pain
Proper diagnosis requires a proper physical examination of the swelling in question. You might consult a gynaecologist who can examine and then decide on the appropriate treatment.
Hope this answers your query. I will be glad to answer the follow up queries that you have.
Wishing you good health.