As Chapter Four opens, why is Cassie moping around the house as if she is sick?

In the opening of Chapter Four, Cassie is moping around and acting as if she's sick because she feels guilty about what happened to the bus. She can't talk about it because she promised her brother she'd keep it a secret.

I think Cassie is moping around the house cuz she doesn't feel like holding the secret of the bus incident, so she feels guilty of herself

She can't share this secret because it might get her and christopher, littleman, and Stacey in trouble, and Cassie promised stacey to keep it a secret or they might get in trouble