Stomach flu is a condition that is caused by the viruses that are different from the ones that causes influenza. It is actually a condition known as viral gastroenteritis and is caused by a number of viruses that infect the intestines. When these viruses infect the intestines, they cause very uncomfortable symptoms that will keep you indoors for days.
On the other hand, food poisoning is normally caused by bacteria that are found in the food that you normally eat. Food contamination can happen at any stage during the preparation and it does not mean that it only gets contaminated when it is ready to eat. How will you know then that you are suffering from food poisoning and not stomach flu? The symptoms of both conditions do not always give the answer because they are similar and both have symptoms such as nausea, chills, fever, diarrhea, vomiting and a times dizziness and lightheadedness due to dehydration.