VSG folks can also experience the symptoms that you mentioned. It is thought to be due to increased gastric emptying, as well as eating/drinking foods or drinks high in sugar or carbs. Another reason to always have protein first, especially with carbs. High carb foods/drinks spike your blood sugar high, so the release of insulin is started to bring down the blood sugar. This spike in insulin starts a cascade of reactions, one of which is the rapid release of fluids in the intestines, which you experienced as the big "D." This is a simple explanation for a more complex series of reactions. It is real and has been documented with VSG. "Dumping or Reactive Hypoglycemia."
Rich foods either in sugar or fat seem to initiate this process. Even people without gastric surgery can experience this reaction, if they eat too much fat and/or sugar. I did experience this prior to surgery on a few occasions, when I ate something with too much fat and/or sugar.