What causes excessive fatigue after a meal?

Possible hypolycemia. You may possibly have reactive or postprandial hypoglycemia, which is a state of low blood sugar due to eating carbohydrate-rich foods triggering excessive Insulin release. You may want to eat more low glycemic, high fiber foods like fruits and veggies, more frequently in smaller amounts to see if that helps. It's always best to see a physician for evaluation.