Why do Cassie feel that "no day in all my life had ever been as cruel as this one"?

After being insulted in the store, Cassie bumps into Lillian Jean Simms, who is with Jeremy and their two younger brothers. Lillian Jean demands an apology, which Cassie reluctantly gives, and then tells her that she should walk in the street. Cassie is trying to keep from being pushed off the sidewalk by Lillian Jean when Mr. Simmons appears and twists Cassie's arm behind her back.Mr. Simmons demands that Cassie apologize to his daughter, although Jeremy insists she already did. Cassie tries to run away and is met by Big Mama. Mr. Simms demands that Cassie say "I'm sorry, Miz Lillian Jean," and Big Mama reluctantly makes her comply. Cassie says it and runs crying into the wagon, thinking this is the cruelest day that she has ever endured. This is why Cassie feels she had the worst day of her life.