sounds like its the change of food to me did you change over immediately? the thing is when changing food its better to put 3/4 of the pedigree food and 1/4 of the new food and increase the new food gradually also treats can give a pup the runs
get an apple put it in the microwave till its all soft and only give one small teaspoon to the pup thats a really good remedy give it for a couple of days that normally works but check with the vet first hope this helps