could be a bartholin cyst

The Bartholin glands are located at the entrance to a woman's vagina, one on each side. They are small and cannot be seen or felt when they are normal. Their function is to secrete fluid onto the mucosal (inner) surface of the labia—the liplike skin surrounding the vagina.

Problems with the Bartholin glands include cysts, which are relatively painless enlargements of the gland, and abscesses, which are infections of the gland. Typically only 1 of the 2 glands is affected.