Since you are not complaining of any pain or fever
and is present for many months,it can't be a boil or folliculitis
The most likely cause should be a sebaceous cyst
Sebaceous cyst doesn't cause any pain unless it gets infected.
You must consult a Gynecologist and get the diagnosis made.
There is no treatment for it except surgical removal of the cyst.
Anti biotics are prescribed,if the cyst gets infected.