A clear bubble on the inside of the mouth is a common symptom of a mucous cyst, according to MedlinePlus. Mucous cysts can occur on the inside of the lips, the floor of the mouth and the gums. The cysts contain clear fluid and cause permanent bumps in the mouth.
Mucous cysts are also referred to as mucocoeles and are caused by clogged salivary glands, according to Healthline. The condition is caused by lip biting, cheek biting and oral piercings. Mucous cysts can also develop if the salivary glands become ruptured. Treatment for the condition is based on the extent of the cyst and include laser removal and corticosteroid injections to minimize swelling. In certain cases, mucocoeles resolve on their own without treatment.