Thanks for asking the query,
Blisters in the mouth can be due to various reasons it can be due to ulcerations, allergic reactions, bacterial and viral infections.
I would suggest you to approach a Dentist
for clinical examination and evaluation which helps in exact diagnosis and treatment.
Apply metrogyl oral ointment topically over the affected area, go for symptomatic treatment at home by maintaining good oral hygiene
to prevent superinfection
, use antiseptic mouthrinse , topical analgesic
, antioxidant suplements.
Hope this helps out.