Marble size,red,painful bump under arm pit for 5 days now quite painful.no other lumps or symptoms. female 37y otherwise healthy.what to do.

Abscess: It is probably an abscess. Sometimes these will settle by themselves, sometimes they need antibiotics, sometimes they need to be cut open to let pus out. If it is quite painful it's probably time to see a doctor

Lump: Most likely this is a tiny boil- an infection that probably began as an ingrown hair. Treatment consists of warm compresses which brings it to a head. If pus drains, pat the area clean, apply antibacterial cream like bacitracin. Don't use any deodorants. Continue these treatments as needed. Sterilize the surrounding skin with rubbing alcohol to kill staph bacteria there. See your physician as need