Stomach bug all night and day, no vomiting and diarrhea, what could be wrong?

Many possibilities: There are many things that can cause gastric (stomach) irritation. The most probable is irritation from something you ate. The alternative is much less likely but depending on your family and personal history can include irritable bowel syndrome, inflammatory bowel disease or other more rare conditions. Start with a healthy diet and regular exercise. Then if still having problems see your physicia.