I'm glad you saw the doctor. It doesn't stand to reason it's cancer, because it's been there for years with no change. When will the lab work be done that identifies it? Please write back and tell what it turned out to be.
As far as the pain it causes, when someone has even a little lipoma [benign fat blob in a sac] in the wrong place, it can be painful. Not saying that what you have is a lipoma, your suggestions of what it might be sound sensible. Just saying that even if something is very basic and unimportant, if it's near a nerve, or digging into a muscle, or in a place where it gets exposed to bacteria, it can be a constant problem. (I've got scar tissue where I broke my collarbone years ago. The bone healed fine. But, the scar tissue is now pressing on a nerve, and it hurts all the time.) There are certainly nerves aplenty in the labial region, and bacteria also is continually available. (Do you wear a 'minipad' and change it every time you go to the loo, to try to minimize any bacteria there?) Anyway, I'm hoping it's something pretty simple and that your doctor can give you good ideas for dealing with it. Please do let us know.