Could my diarrhea, abdominal pain, excess gas, and very smelly poo be caused by eating too much sugar? My stool culture and parasite check came negetive.

Continue workup. Despite the "pop" wisdom, sugar in your diet won't do this. It's not helpful nutritionally and tends to make you fat and is rough on the teeth. If I were seeing you, I'd test you for gluten enteropathy, and if you don't have that, then Whipple's, pancreatic insufficiency, and a bunch of others would be considerations. Are you losing weight?