Why was Cassie mad in chapter 5

Cassie is angry for a few reasons. It has been almost an hour, and even though Stacey and TJ try to stop her, Cassie marches over to Mr. Barnett to remind him that he has forgotten about them. He responds angrily, telling her to get her "little black self" back to waiting and calling her a "little nigger." Cassie protests, until Stacey begins to drag her out of the store, and Mr. Barnett tells him to "make sure she don't come back till yo' mammy teach her what she is." Cassey is also forced to apologise to Lillian Jean for just being on the sidewalk.