I keep getting clear blister in my mouth. what could it be?

Clogged mucous gland: most like this caused from a recurring clog in one of your minor salivary glands as it can plug with mucous at times. Sometimes it is necessary to remove the small area of tissue to prevent recurrence.

Oral Mucocele: Mucoceles may go away on their own in 3–6 weeks.or may be treated by your doctor by being carefully cut out. See your doctor if the bump persists for over 2 months or if it is bleeding. Take care.

Mucocele possibly: This bump inside of your mouth is probably a mucocele. It is caused when the duct to a minor salivary gland gets clogged or traumatized and the saliva backs up forming a fluid filled bump. They are most common on the lower lip but can occur elsewhere in the mouth as well. Small ones can resolve on their own. Large ones (as shown here) require surgical removal by and oral surgeon.