Thanks for the query.
Sudden appearance of water filled blister
suggests of herpes
It is a viral infection and is associated with pain and burning.
It usually comes after a stressful event like fever or mental tension.
Apply oint acyclovir
5 times a day.
You will get some relief.
Please meet a dermatologist
for exact diagnosis.
Let me know if you have any other doubt.
you can ask a direct question to me on this forum, following the below link.
Wishing you a good health.