Why do I feel so tired after eating?

You shouldn't. After a meal, especially a large meal, they body diverts more energy to the intestines to digest the food. Energy now not available for other purposes. Also, meals rich in refined carbs (sugars and starches) may cause a rapid rise in blood sugar then increased Insulin output and a subsequent dramatic drop in blood sugar. That may also cause fatigue. Healthier meals should not cause fatigue.
Maybe food allergies. As drs. Beede & wyatt mention, changes in blood sugar could be to blame, but fatigue after eating is a common symptom of food sensitivities. Adverse reactions to foods are extremely common & can cause fatigue, pain, GI distress etc. The most common foods people react to are dairy, gluten, eggs, corn, soy & almonds. Tests are imperfect, but you can try eliminating all for a week & add back 1 at a time.
Carbohydrate effect. Carbs can cause a "tired" feeling in some around 1-2 hours after eating a meal. This can be a sign of hyperinsulinemia- which is associated with metabolic syndrome and occasionally pre-diabetes and hypoglycemia. Done basic lab work- fasting with metabolic panel, cholesterol levels and fasting Insulin can give good information.. Be wary of simple carbs...Ie white sugar/ candy, desserts...