This could be from bacteria from the mouth or it could be an allergic reaction to toothpaste.
It can be a mucocele, a blockage of the salivary gland of the mouth or a small benign growth. It 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.It then has to be differentiated from other mucocutaneous lesions also.
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.
Thank You so much doctor, that's lot of help. It matches exact description what you have posted in your comment. I had visited doctor even he said, it has happened because of cheek bite or trauma while eating the food. He said no need to worry it will go on its own, but it will take some time.
Thanks for the comment, I have switched back to my old tooth paste.