i had one of those about 2 years ago. although i was stupid and didnt go the doctors about it. i waited to see what would happen... thats stupid so dont do that. it did go away by its self but everyone is different and i wouldnt gamble with your vagina/area. just go to a doctor and they wil most deffintely figure out what it is. good luck! :-)
I think you can also get varicose veins in that area. Best to have a doctor check it out.
njeosys is right, i've had a few of these, and i've been tested for any kind of std pretty much, but actually.. mine was on the outer part where the pubic hair was and the doc actually told me it was from shaving and that ingrown hairs can create cysts and it's somewhat common and that made me feel a lot better.. but if it's on the inner labia, i wouldn't really know about this. you'll have to go to your doctor.
It could be a benign cyst. You'll have to make a doctor appointment and have them look at it. Some cysts disappear on their own, others needs to be surgically removed.
It could also be an STD of some sort. That is also why it is important to check with your doctor. Are you pregnant?
have you been with anyone unprotected in the last 2 weeks?
i dont know have you showen it to your doc i think that would be wise