Is it true that reheated food if not heated well can cause a stomach flu?

No, reheated food cannot cause stomach flu. Stomach flu, also known as gastroenteritis, is caused by highly contagious pathogenic viruses or bacteria. Heating and reheating food to an internal temperature of at least 74°C (165°F) kills most foodborne pathogens. Foodborne illnesses result from consuming contaminated food or beverages, not from reheated food that has been thoroughly heated.