I would guess it's because our intestines are far longer then the intestines of a carnivore.
When a carnivore eats meat he has strong intestinal acids that digest the meat and short intestines to quickly eliminate the meat.
We don't have those strong stomach acids and far longer intestines. So meat, dairy etc. starts to rot inside our body because it stays in there too long. This creates all kinds of toxic side products and creates a nasty smell.
Maybe you have noticed going to the bathroom also smells a lot less now? Or maybe day 4 is still a bit to soon.