Brief Answer:
most commonly Hidradenitis

Detailed Answer:
Hi.
Thanks for your query.
Noted the history well.
Bumps in armpits which produce pus after hot compresses is usually due to a condition called Hidradenitis Suppurativa.
I would advise you the following:
Please send the clear and focused Photos.
Please give additional information:
Since when is the problem present?
Is this recurrent?
Any medications tried for this condition?

Clean the area with plain water as many times as possible.
Keep dry with a cotton cloth or hair dryer.
Apply Liquid Povidone Iodine locally once or twice a day and allow it to dry up - helps sterilize the area.
Avoid hot and humid atmosphere.
Use pure cotton garments.
Consult your Doctor to get the clinical evaluation, confirmation of the diagnosis. to get a valid prescription of an antibiotic and anti-inflammatory.

I hope this answer helps you; please feel free to ask for further relevant queries if you feel that there is a gap of communication.

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