I too have this syndrome. Lightheaded, fast heart rate, no energy, abdominal pain, diarrhea. Researching Dumping Syndrome, I find that these are classic symptoms and they an occur immediately or within 1 hour or be delayed and have an onset of 1-3 hours after eating. Common triggers are rich foods, high sugar foods, large portions of food, and having food and drink together.

I'm more susceptible to this because they did a pyloric myotomy during my gastroesophagectomy to prevent delayed gastric emptying.