Well it depends. Are you a boy or a girl? If you are a girl, it could be your period. If you are pregnant, Your belly button might pop out any second. Or you could have scratched it. Did you dig...
If there is a bulge near the belly button (and not the stomach which is slightly higher) then I would say it is a hernia, and you can google umbilical hernia to check what it says. A lot of babies...