Probiotics are brilliant. Probiotics and a few days with a diet specifically made for troubled stomachs (both can be purchased from vet office), can save you a consultation with the vet in man cases. Just be sure to gradually swap back to normal food over several days when the stomach is stable. And remember that puppies should not go many days with diarrhoea before you seek medical advice, they get dehydrated easily when they are little. And remember to make sure appropriate vaccinations and deworming has been given as well as a suitable diet for that age.