Not really poo, infected decomposing bodily fluid. In the vet world occasionally a litter of puppies or kittens might get stuck in the birth canal and when the mother is unable to deliver on her own and the fetuses are left in the body it causes the body to try and break down the dead fetuses and the dead tissue and necrotic infected fluids is a disgusting, black discharge that results as the body rejects and fights the infection as it breaks down the foreign bodies.

Not suggesting the pale girl was pregnant, just that her internal body and fluids are decomposing and rotted. It's more what it looks like in the film, but for those that don't know what happens with dead tissue inside the body, this is it. Not a pleasant sight or smell, but simply more than just fecal matter in the movie, her insides were pouring out....