Stomach pain with vomiting and diarrhea is often due to Gastroenteritis. The most important treatment is fluids, take ORS frequently to prevent dehydration, eat light foods like rice and curd, and rest. You can use Ondansetron for vomiting and probiotics for gut recovery if prescribed. Avoid self-starting antibiotics. If symptoms are severe, there is high fever, blood in stool, or dehydration, consult a doctor immediately.