I cannot confirm anything without examination but it can be a canker sore or mucocele. Canker sore is painful but mucocele is painless. Mucocele 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.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.