it can happen in all breeds i had a shih tzu litter and they were all healthy in the morning and than in the afternoon the one puppy was not as strong would not nurse very limp when you picked him up so i went out and bought pet tonic which is like a stimulant and gave it to the puppy and fed him some milk by hand to strengthen him up and it took a couple of days and he got his strength back he was always a little smaller than the other puppies but he survived If you keep a close watch on the puppies you can see the change in the puppy