its a Hernia....
I've had the exact same thing for a couple years now....figured it was a hernia and now confirmed by a physical......belly-button pops out and can push it back in.......the doctor wasnt even concerned, as I was because I'm not about to give up lifting heavy, anyway he said just make sure to exhale when lifting and that these type hernias that appear in the navel about the size of a dime arent a major concern.....
I admit, now that its confirmed I have one, I am extra careful when I get crazy heavy.....
Peace,
H-man