Soda is extremely acidic—think battery acid levels, but safer. Phosphoric acid and citric acid are common.
Your gut lining isn’t made for that pH punch. It can get irritated and inflamed. An irritated gut reacts by flushing itself out.
Congratulations. You just turned your stomach into a fire hose. Acid + fizz = bathroom visit.
Funny detail: Some people can drink a whole two-liter without issue. Others get the runs from a single can. Blame your genetics.