Gregor's mom found Gregor and she was scared by him.
Grete gave Gregor a bowl of milk because she knew that it was his favorite food /drink.
Grete noticed that their was only a little bit of milk spilled, so she figured that he didn't like the milk anymore. Grete got a piece of newspaper and put on a some vegetables and a few bones that were left over from the dinner.
This when Gregor's father is throwing apples towards him and an apple gets stuck between his wing and his body, he was in a lot of pain.