The upset stomach after eating breakfast may be caused by dietary factors, or it may be caused by indigestion, acute gastroenteritis, gastric ulcer and other disease factors. 1. Dietary factors: eating spicy and stimulating food or overeating at breakfast may stimulate the gastrointestinal tract, resulting in an upset stomach. 2. Indigestion: when indigestion slows down gastrointestinal peristalsis, there may be a feeling of early satiety after eating food, which may also be accompanied by bloating, nausea, vague pain in the upper abdomen and other symptoms. 3. Acute gastroenteritis: If you eat spoiled, unclean food for breakfast, it can lead to bacterial and viral infection of the gastrointestinal tract leading to acute gastroenteritis, abdominal pain, diarrhea, nausea, vomiting and other symptoms. 4. Gastritis, gastric ulcer: gastritis, gastric ulcer patients can have symptoms of postprandial pain, because after eating gastric acid secretion increases can stimulate the gastric mucosa, resulting in pain. It is recommended to go to the hospital if you have an upset stomach after eating breakfast.