It`s a Mucocele.
An oral mucocele is a harmless mucus-filled cyst on the inside of your mouth. The cysts are usually painless and look like smooth, round lesions. Trauma such as biting your lip cause most oral mucoceles. You typically don't need treatment because the cyst should go away by itself.
But if it bothers you much,then there are various treatment options:
-Surgical excision.
-Laser treatment.
-Cryotherapy.
Next Steps
Visit a local dentist and get it clinically evaluated.