hello thank you for trusting HCM dear it is very difficult to say what is cause until unless examination was done.here I mentioned some of the causes for blisters in mouth with pain. Recurrent aphthous stomatitis (canker sores) ,viral infections (particularly herpes simplex and herpes zoster),other infections (caused by fungi or bacteria),Injury or irritating food or chemicals Tobacco use,drugs (particularly chemotherapy drugs) and radiation therapy,systemic disorders etc... use mouth wash ,avoid spicy food and alcohol.use dentogel for local application,use zincovit once a day.if symptoms not improved please consult your doctor he will examine and treat you accordingly. I think I answered to your question if you have more questions feel free to ask. take care