I'm embarrassed to not know this properly considering that I'm a medical student in their fifth year but tolerate me for a sec. We have two main things that can cause your cramps. It can either be preformed toxins which are seen in spoilt food and responsible for food poisoning, or it can due to viruses and bacteria which may have certain enzymes or proteins that causes Chloride channels in the lining of the G.I. tract, I believe it's the intestine (I think) and I believe there's reduced reabsorption of water.(Usually water you ingest is reabsorbed in the small intestines along with other nutrients and minerals) Generally people confuse food poisoning and stomach flu but there's a lot of differences like food poisoning lasting very briefly and being caused by pre formed toxins while stomach flu being caused by pathogens like bacteria, viruses, fungi etc.