Karla is right to suggest that food is the most likely cause, and this is likely to
be dairy stuffs. That said, one of my prior dogs
was found to
be allergic to
animal fats, and once we had identified it we started feeding her nothing but white fish and chicken with rice, which ultimately resolved all the problems. Many dog treats include animal fats, so we had quite a job finding things she could have as a special treat, but we did eventually succeed (we fed rice bones, which she loved).