HI.
Thanks for your query. 
Read and understood your history of mastoid operation 15 years back. You has a bath the way you explained it. 
You developed pain in the right ear, to brain nerve at forehead, and cleaned the ear with 
hydrogen peroxide. the pain still persists. 
The ENT must have examined everything and found nothing specific in the ears hence he prescribed you 
vasograin in the hope that you are suffering from 
migraine. 
This is the medicine of choice in cases of migraine alone and not effective for any other disease.
I would advise you to continue the same in the hope to get a relief.
If not, Consult another ENT for 
second opinion and then a 
Neurologist for clinical evaluation, examinations and investigations like MRI of the brain, mastoids and sinuses. 
This will help in diagnosis by either confirming or ruling out any disease in particular and help in further management .