I have an Italian Grayhound, I also work at a pet store(so I am knoweledgeable however my advice is just that, advice. I am not a vet)
Anyway my pup is allergic to grass in Spring and Summer she will chew on her paws after she goes outside. Many dogs can be allergic to grains or meats in the food, it's best to choose a higher quality food rather than something like pedigree. While switching foods there is a canned food which will help settle a dog's stomache. It's called fruitables pumpkin. The only ingrdient is pumpkin and it helps setltle a dog who has consitant diarreah.
As for yeast infections you can give your dog a VERY small amount of plain yogurt. It helps balance out the yeast in a dog's body but don't give the dog to much it'll cause upset stomache. Also, do not give it daily or anything.
If the dog still is having diarreah which can't be controlled. Switch the dog to boiled chicken and plain boiled rice for a few days until you see solid stools again. Then swich back to regular dog food.