This seems to be blister of oral mucosa.
Blisters arising in the mouth may be due to the autoimmune disease , pemphigus and bullies pemphigoid. These are characterised by autoantibodies directed against the adhesion molecules of the skin and mucous membranes.
Next Steps
Treatment may also include:
Topical steroids
Tetracycline / nicotinamide
Azathioprine
Cyclophosphamide