My belly button has popped too...of course I am 7 months pregnant, and somehow I suspect that is not the case with you. May I suggest the men's health forum? This is a pregnancy forum so I don't think you will get any answers here. Sorry.
Discuss this with your doctor. A morphological change in your bellybutton warrants medical investigation.
You know...it's not the right forum...but I can tell you that the same thing happened to my dad. He said it wasn't painful, but it got to where it was like his belly button was turned inside out. Turned out to be a hernia.