Diarrhea and vomiting are the most common symptoms of an upset stomach in dogs. Here are some warning signs to keep an eye out for: 1. A growing sense of unease. 2. Vomiting or diarrhea more than two times in a row. 3. Vomit or feces with blood. 4. Vomit or stool with a toy or other foreign object. 5. Defeat or failure.