Hello dear
Understand your concern
Exact diagnosis made by examination, you can send me the picture.
I think it is herpes infection, because of painful and spreading nature.
Consult the doctor and get done test for herpes like
herpes simplex antigen and antibody by Elisa.
Other causes may be trauma, infection, allergy, allergy, immunological disorder like bullos
impetigo.
Advice:
Do not pop the blister it can cause infections
After brusting you can wash with water and then
Apply bandage till it heals
Oral
acyclovir tablet
Apply acyclovir/valcyclovir ointment
Also apply
neomycin ointment to prevent infection
You can also apply calamine lotion
Hope this may help you
Best regards
Dr. Sagar