The picture is in my photo album.
It looks like a plugged up abscess or boil.
It seems to have a scabbed top and looks like a plug of pus underneath that is about ready to erupt. The doctor may decide to lance it to get rid of the rubbish.
He may prescribe an antibiotic if he feels that this is required. It is important to get all the "core root" out. If any is left, it will fester and come back again.
Make an appointment to see the doctor.