What was the screaming in the middle of the night in The Secret Garden?

At the beginning of chapter 17, Mary is awoken by "such dreadful sounds" in the middle of the night. She hears someone "screaming and crying at the same time . . . in a horrible way," and the screaming is also accompanied by the sounds of doors being opened and shut and the sound of feet hurrying through the corridors.
Mary at first panics, but she soon realizes that the screaming is coming from Colin, who is having "one of those tantrums." She is used to Colin having tantrums, but this one seems especially bad. Mary puts her hands over her ears to block out the noise, but the screaming is too loud, and Mary becomes angry. Colin only stops screaming when Mary goes to his room and starts shouting at him, telling him, "You will scream yourself to death in a minute, and I wish you would!" Colin subsequently reveals that he was screaming because he thought that he had felt a strange lump on his spine. He was so terrified because he was afraid that he, like his father, would grow up to have a crooked spine.