It may either be erupting last molar which is compressing upon the nerve
supplying the oral cavity. Kindly let me know your age.
Or it may be Folic Acid deficiency
. You may take
Tab. B-FOLCIN (FOLIC ACID) 5 mg twice a day.
Use a mouth wash like HEXIDINE gargles four-five times a day.
Cover the blister
with pain-killer gel like MUCOPAINE before taking food. Do not worry about MUCOPAINE going into stomach as it is medicine.