Hi
Welcome to DoctorSpring.
From you description it seems you are having a furucle. Its is an infection of hair follicles causing a pus collection and swelling. If not much troublesome it can resolve by itself. However if you have diabetes or if you have depressed immunity you should contact your doctor. If the swelling is persisting for long, if there are fomation of new such swellings, or if there is continous pus drainage from the swelling, get medical help.
He may prescribe you a course of antibiotics and if needed an I&D - Incision and drainage to let out the pus.
Meanwhile maintain good personal hygene. Use bactercidal sops. Keep your armpit dry after bath. Take a shower inbetween if you sweat a lot.