Hindi ko pa naririnig yan sa mga kaibigan ko, but honestly it could be normal depending sa weight changes or skin elasticity, like kung bumaba ang weight mo or nag-exercise ka, sometimes ang skin around the belly button area just adjusts. If it's not causing you any pain or itching though, I wouldn't worry too much about it, pero if it keeps changing or may discharge or something, better check with your doctor just to be safe!