Brief Answer:
Can be a Bartholin's cyst.

Detailed Answer:
Hello,
Thanks for posting your query.
The causes for a small lump underneath the skin of the labia Majora could be one of the following:-

1. Bartholin's Cyst - Is located midway of the labia, and protrudes into the vagina and causes 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).

If infection sets in, the result is a Bartholin's abscess. If the infection is severe or repeated a surgical procedure known as marsupialization may be required to stop further recurrences.

2. Infected hair follicles - can form small absesses, which can cause a small swelling, but they are 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.

I hope this answers your query.

In case you have additional questions or doubts, you can forward them to me, and I shall be glad to help you out.

For future query, you can directly approach me through my profile URL http://bit.ly/Dr-Rakhi-Tayal

Wishing you good health.
Regards.
Dr. Rakhi Tayal

---