Thanks for writing your query.
It seems that the swelling in your armpit is having pus collection, which is causing intense pain.
The best management will be to get it incised and drained by a small surgical procedure under aseptic conditions.
You can apply locally Sumag cream (Magnesium
Sulfate) on the swelling for few days. Taking pain killers like Ibuprofen
one tablet twice a day after meals along with antacid Omeprazole
20 mg will give you some relief but your pain will disappear only after drainage
of pus. Antibiotic may be needed to cure the infection completely.
Take medicine under medical supervision only.
I hope this is helpful to you.