Maybe its a Bartholin gland cyst. Here is some info from this website: http://www.familydoctor.org/handouts/235.html
What is the Bartholin's gland?
The Bartholin's gland is a tiny organ on each of the labia (vaginal lips), near the opening of the vagina. If the vagina were the face of a clock, these glands would be found at about 4 and 8 o'clock. Normally they are invisible. They make a small amount of fluid that lubricates the vaginal lips. If a flap of skin grows over the opening of one of the glands, the fluid backs up. It causes a round swelling called a cyst (say "sis-st"). The cyst can grow from the size of a penny to larger than an orange, although most don't get bigger than a golf ball. They can be tender.
Are Bartholin's gland cysts caused by an infection?
Most of the time, Bartholin's gland cysts are not infected. In some cases, however, they can be caused by an infection, or they may become infected. Your doctor may want to check the fluid in the cyst. Most infected cysts, called abscesses (say "ab-sess-es"), contain the normal bacteria that are found on your skin. Some infected cysts, however, are caused by sexually transmitted bacteria (germs).
How do I know if I have a Bartholin's gland cyst?
You may notice a round, painless or slightly tender bulge in one of the vaginal lips, near the opening of the vagina. It may stay the same size or may slowly grow larger. Cysts that get infected are usually very tender. In extreme cases, walking may be painful. Your doctor will look at the area to see if you have a cyst and to find out if it's infected.
Hope this helps!!!