Jemima the Pig and the 127 Acorns by Michael Morpurgo

When a young boy goes from the city to stay on the farm, he discovers a new world - one where ducks and geese roam, sheep graze in the fields and apples grow in the orchard.
 
There he meets Jemima the Pig. She smells and she slobbers, she breaks out of her pen to steal potatoes and she's definitely the queen of the farm. Soon, he's hiding acorns in his pockets to feed to her. But then Jemima starts to feel ill. Could she have eaten too many acorns? Or does she have a secret?