Early signs of a stomach infection—commonly known as gastroenteritis—typically come on suddenly, starting with an uneasy feeling of nausea, a loss of appetite, and mild abdominal rumbling or cramping. As your immune system reacts to the invading virus or bacteria, you may experience a low-grade fever, chills, and an unexpected surge of fatigue or muscle weakness. These subtle initial warnings are usually quickly followed by loose or unusually soft stools, which often progress into watery diarrhea as the infection takes hold.