Going roughly by the description that you have mentioned it seems that you have a mucocele on the lower lip. This is a condition which does not Heal on its own and neither can be treated by medications. It is a balloon like cyst below the skin surface and the only objective treatment is the removal of this entity.  It might show reduction in size when it gets punctured by any trauma but it is going to refill on its own to achieve the same size back and this will continue periodically unless treated. Regards.