Could be umbilical or para umbilical hernia , means u might have a defect in ur abdominal wall of muscle through which inner content come out as swelling... Does the size increases with cough ? If yes u might have hernia ... Does it reduces when u sleep ? Can u push the swelling inside with ur fingers ?? If u u can't push inside and it is doesn't go inside when u sleep means , there is chance that the Hernia might have obstructed... Abdominal pain ???? Are u suffering from constipation too ?? If yes better first meet the local surgeon.. Could be a sign of obstruction...

It may be due to umbilical/paraumbilical hernia....get yourself checked from a surgeon, or send a photograph of your naval when swollen.