In chapter 7, why did Brian wake up screaming?

Brian screamed because he was sick and in great pain.

"MOTHER!"
He screamed it and he could not be sure if the scream awakened him or the pain in his stomach.
His whole abdomen was torn with great rolling jolts of pain, pain that doubled him in the darkness of the little shelter, put him over and face down in the sand to moan again and again: "Mother, mother, mother..."
Never anything like this. Never. It was as if all the berries, all the pits had exploded in the center of him, ripped arid tore at him. He crawled out the doorway and was sick in the sand, then crawled still farther and was sick again, vomiting and with terrible diarrhea for over an hour, for over a year he thought, until he was at last empty and drained of all strength.