I also have a painful belly button area :)
My seems to be the area around my belly button and not the belly button itself...strange. I usually attribute it to stretching, but sometimes I feel something hard in one spot, and at first I thought maybe it was a hernia because it was painful when I pressed it; however, the hard spot responds to prodding and moves, so apparently its baby!
I am having twins, so it is very hard sometimes to know where each one is and what part of their bodies is where lol!