It sounds like you have a boil in your armpit. A boil is caused by a bacterial infection within a hair follicle or oil gland; the most common bacteria found in boils
. As pus collects within a boil, it grows larger and develops a "head" that may eventually burst and drain. You shouldn't squeeze a boil, as this could spread infection to surrounding tissues or even into your bloodstream. Apply warm packs to your armpit and call your doctor. You may need an antibiotic to get this problem cleared up.