You have developed an Abscess
on your under arm which has ruptured on its own.But its likely that the Infection is persisting and can spread to the surrounding area.Depending on the size of the abscess you will have to get it drained by a doctor at the emergency department.
Warm compresses around the abscess might help but do not press on it as it might drive the infection deeper.
Visit a doctor and get it drained and they can also advise you regarding need for antibiotics depending on the size of the Abscess.Also please note that you can develop fever
as part of this infection and its wise to visit a doctor especially if the area is looking red and angry.
Hope this helps!