How can i tell if i had food poisoning or the stomach flu? also can i reinfect myself ? (i didn't take any antibiotics)

Gastroenteritis: You usually don't reinfect yourself with the same bug. I don't know exactly what you mean by stomach flu but most gastroenteritis resolve on their own, we just have to make sure that we stay well hydrated during the episode. If diarrhea persists for a long period of time or if there is fever or blood, make sure to consult with your doctor.