Hello,
It can be a canker sore( painful) or mucocele( painless). Mucocele is a rubbery, bubblelike painles swelling. It is usually inside the lower lip,under the tongue or the inside of mouth and caused mainly by biting the lips or trauma during eating of food.
Usually it goes away on its own. But if it does not, then corticosteroid injections and surgical removal by scalpel or laser are done by a dermatologist.
I suggest you to get an evaluation done from a dermatologist.
I hope it helps. Take care and regards.
any help please its now grown even bigger