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(view spoiler)[In the book, Abby says that her stomach pains were a problem and that she wouldn't tell her mother about the pains so that "Mom would be sorry". (It's after she finds out that her mom wants to sell the house I think) I don't think it's mentioned in the book what the nausea and pains were... Any thoughts? (hide spoiler)]
Autumn Abby waited to tell her mom because she was mad at her. SHe decided to wait until it was too late so that her mom would be sad that she didn't listen to Abby. In other words, Abby was being selfish. If you are asking why she got the pains in her stomach it's because many people experience grief differently. Some people experience grief in being sad all the time. Some shout. Abby bottled all her emotions up inside of her and that is why she had stomach pains she wouldn't let herself express any feelings so her body deided to cope with it another way.