The sharp pains the stomach gave her, it is like fighting back against a person. Rein think to herself, "I don't want to go to school." However, she remembers that her mother said that if she does not go to school, She'll whack Reim's ass until it blooms flowers. Rein, at the moment of thought, shiver. 

The stomachache is what keeps Rein not wanting to go to school.