Mistress Melis has a sensitive stomach and her chef knows it. But he had prepared food she did not like and that her stomach could not process. She ended up having diarrhea and she did not want to suffer alone. As he had started it all, he had to share in her misery and that is why she chose to make him eat her shit as well. He had to eat her diarrhea and he also had his own diarrhea after that.