Your belly button smelling like poop is most likely caused by a fungal infection that can be cleared up using some fungal cream and washing out your belly button.
You can also clean the belly button using some rubbing alcohol or vinegar which helps to kill the bacteria causing the fungal infection.
Some other causes of a smelly belly button and a belly button that smells like poop is due to an unclean belly button that has dirt and bacteria, and other germs inside it.
Even when you shower sometimes dirt and germs and that other bacteria may still reside in the belly button especially if you don't clean it good enough.