Hi,
Welcome to the ehealthforum and I am really glad to help you out. The likely possibility is a Bartholin's cyst develops when the duct coming out from the Bartholin's gland becomes blocked.
The fluid produced by the gland then accumulates, causing the gland to swell and form a cyst. The treatment consists of the surgical removal of the cyst. Other possibilities which need to be entertained are lipoma, sebaceous cyst, genital wart etc.
A gynecologist examination is mandatory as it is not possible to make an online diagnosis. I sincerely hope that helps. Take care.