If it's like a blister or cold sore, it's likely herpes. If it's more of a wart, a painless bump that is lighter in color than the surrounding tissue it's probably a vaginal wart caused by human papilloma virus.
If it's painful, like a cyst or pimple it's likely that it's a bartholin's gland cyst. The bartholin's glands provide lubrication and moisture for the vagina. There is one on either side of the vaginal opening. They can get blocked and cause painful cysts which will go away.
You should see your doctor about any of these conditions. Hpv can go on to cause cervical cancer. And herpes can be transmitted to your lovers.
Take care.