It sounds like it might be too huge to be one, but could it be a sebaceous cyst? I have gotten pretty large ones, but none that big.
Sebaceous cysts feel like marbles under the skin, but it is a pocket of sebum usually made by irritation, like friction or an infected hair follicule. It is basically a huge pimple, and they can move around (as in, they are moveable, but don't migrate

).
If it does have a head, the only way you can get rid of them is popping it until you reach blood, keeping it clean, and using an acne cleanser or cream to make it go down. They usually reappear as they do spread. Sometimes you have to have the sac removed surgically. They are not contagious, and it is a normal thing to have sebaceous cysts.
Also, if your gyn didn't say anything about it at your last exam, I would think you are ok. They are supposed to check things out pretty well down there. If he or she was concerned about it I am sure you would have had a discussion.
I wish you the best!