I seem to have ,maybe once a month, terrible gas,that smells like sewage, followed by the nwxt day or two with horrible diarrhea ?

One of the first: things we think of with foul smelling flatus is malabsorption: gluten sensitivity, inflammatory bowel disease, carb/lactose intolerance, food allergies.But the cyclic nature of your problem suggests persistent infection to me...? giardia. I would consult with my doctor and consider stool cultures for parasites/bacteria.Other thoughts: chronic pancreatitis, high dose vitamins/supplements. Good luck