Why did Big Ma think that Cassie was sick? Explain.

This actually takes place in Chapter Four. The chapter begins with Ma questioning Cassie about why she has been moping around. She thinks Cassie is sick because of the way she is acting...... or maybe Cassie just has something she'd like to tell her like the bus incident.

'Nothin' !' Big Ma turned and looked directly at me. ‘You been mopin' 'round here for the past week like you got the whoopin' cough, flu, and measles all put together." ...."Mama studied me with the same disturbed look Big Ma wore and a tiny frown line appeared on her brow. 'Cassie, she said softly, fixing her dark eyes upon me, 'is there something you want to tell me!'"