Hello,
The most likely cause is an umbilical hernia, but the only way to tell would be an examination at your provider's office or a clinic. Umbilical hernias are caused by a protrusion of bowel through the musculature surrounding the umbilicus (the belly button).
http://hernia.tripod.com/umbilical.html
Nurse Practitioner
Board Certified Nurse Practitioner, MS, RN. Private practice and hospitalist experience.