This is due to a small infective polyp deep in the umbilical cavity.
Try to clean the umbilicus
with boiled and cooled down water and a sterile Johnsons ear bud.
After that you can try application of some Potassium permanganate crystals on the infected area deep inside.
Wash it out after an hour or so.
if it does not respond in two or three days, show your local surgeon.