In a normal person when they eat lots of hydrochloric acid is produced in the stomach to liquify the food. The strong acidity also triggers the iliac valve, the valve between the small intestine and the large intestine, to open and release the contents so the person feels the urge to have a bm within a hour after eating. For you it seems there is enough acid in the coke to trigger the iliac valve. So it is likely that you are not producing sufficient stomach acid. This may eventually result in a low vit B12 level and some other problems. Some people can improve their digestive system so that their stomach acids return to normal, but older people may have to supplement with B12, Folic acid and Niacin and supplementing acids with hydrochloric acid drops or Betaine HCl. It might help to have a test for B12 level and if low have a test for Pernicious Anemia.