Hi,
Welcome to Just Answer.
It looks you are having a condition called umblical hernia.It is just a benign condition that develop because of weakness of muscles at belly button and protrusion of abdominal contents occur through it.Usually it stays the same size many years but if it increases in size then you should worry.Just a very small operation is done in which that weak hole is closed and thats it.See this link also:
http://en.wikipedia.org/wiki/Umbilical_hernia
Regards,
Dr.Naeem
Doctor (MD)
Medical Doctor,Ireland.