Hi, and thanks for your medical question on eHealth forum!
What you have is probably a Bartholin cyst. On each side of the vaginal opening there are Bartholin's glands which secrete fluid to lubricate the vagina. At times their openings become obstructed causing fluid to accumulate in the gland, resulting in relatively painless swelling called a Bartholin cyst.
You can soak in a tub filled with a few inches of warm water (sitz bath) several times a day for three or four days. This may rupture cyst and which will then drain on its own. If this does not help, then please consult your gynecologist for incision and drainage.
It can also be genital wart, or a polyp. Please do not prick or rub against it. Since a confirmed diagnosis cannot be given on net please consult your gynecologist regarding this. Hope this helps. Take care!