In "Tom Sawyer," why does Sid become suspicious Chapters 9-17?

Sid becomes suspicious when Tom has problems sleeping at night. He becomes doubly suspicious when Tom begins to talk in his sleep.

"And you do talk such stuff," Sid said. "Last night you said, 'It's blood, it's blood, that's what it is!' You said that over and over. And you said, 'Don't torment me so—I'll tell!' Tell WHAT? What is it you'll tell?"