Bartholin duct cyst
Two glands, one on each side of the vaginal openning known as bartholin glands produce lubricating fluid important during sexual arrousal.
The fluid is transported from the gland throught a tube known as the bartholin gland duct, which due to various causes of inflamation, becomes plugged. The duct then expands filled with the trapped fluid. Drainage is needed. See your gynecologist