Why is my dog whimpering every time I pick him up?

Hi my dog is a 9 week old chocolate lab puppy and he whimpers when we pick him up. We pick him up by under his arms--like his arm pits. He has never done this before bu he may be tired but we just gave him a half a cup of his food and he scoffed it down like he usually does. We don't know what to do but we felt his body and it seems fine and now he is walking around like usual. Is this normal? Please help!