There are many potential causes for upset stomach in puppies. They love to explore their new environment through their mouths, which can lead to ingestion of non-food items causing stomach upset. Sudden vomiting and diarrhea may also be caused by a new food or treat, diet change, a blockage of the digestive tract, intestinal parasites, infections with bacteria or viruses, exposure to toxins, or disease affecting other organ systems. Some cases are mild while others can be severe to life threatening. It’s best to work with your veterinarian to determine the cause for stomach upset in your puppy.