Without an exam I can't say for sure but the most likely cause is a bartholin cyst which results from fluid filling the duct that leads from the bartholin gland - the organ that lubricates the vagina. If this is the case, it is harmless. You can see your doctor to be sure and often they can be removed but draining often results in their return. There are other possibilities such as infected follicles, other cysts, polyps or warts so for an accurate diagnosis you should see your doctor.
The answers in online forums are for information only and are not intended to substitute for medical advice and care. Please see your personal health care provider for further evaluation of your individual case.
Debbie Miller, RN