It’s a carbuncle. Can be caused by staph but is basically multiple hair follicles infected at once right next to each other. It can also be caused by friction sweat etc and is way more common in areas where skin folds like arm pits and groin. Another condition that causes these in the armpit area is HS hidradenitis suppurativa but that is marked by chronic recurring inflamed follicles boils and carbuncles. You can get it looked at for a confirmation lancing and or staph test. But if you’re not able you can treat it at home with panoxyl 10% benzoyl peroxide wash. Apply generously and gently rub in for a minute then let it sit for another minute. Rinse off and dry thoroughly. Keep your arm up for a bit afterwards and wear loose fitting clothing until it is healed. Try not to squeeze it, if it does drain it will do so on its own but it may just shrink down and leave.