One possible cause is a problem with blood sugar levels. If the foods are the type that the body quickly metabolizes, your blood sugar level can take a dramatic dive after an initial rise. This is worsened if you have diabetes or pre-diabetes - which many people have but do not realize. Another reason is problems in the circulation system which can be stressed after a meal, or with the liver. Best to get checked out by a doctor.
While correct, for most animals, the resulting blood sugar changes trigger an insulin release. I find eating a more protein rich lunch, cutting out high glycaemic carbohydrates prevents me from having problems with tiredness.