Although it's commonly called stomach flu, gastroenteritis isn't the same as influenza. The flu (influenza) affects only your respiratory system — your nose, throat and lungs. Gastroenteritis, on the other hand, attacks your intestines, causing signs and symptoms such as: Watery, usually nonbloody diarrhea — bloody diarrhea usually means you have a different, more severe infection Nausea, vomiting or both Stomach cramps and pain Occasional muscle aches or headache Low-grade fever