Goddess Nemesi was fed up with the way in which this guy acted and she wanted to put an end to his nonsense. That is why she chose to use her shit to punish him and to teach him a lesson. She had to do it for fun as well and so she combined all those reasons and she had fun at his expense. He never messed with her again.
Goddess Nemesi had started seeing this guy and it was all going well until he forgot her birthday. She had told him a few times and he told her that he would make it special. The fact that he forgot about it made her feel not appreciated. She had done something special for his and so she had to make him pay for his mistake. That is why she had him eat her shit.