Allergies can do this. It could be due to many things as the above posters have said. I would ask the doctor for a food allergy blood work panel to see if you indeed to have food allergies, if you do, then you can avoid those foods and you will probably start to feel better.
It has happen with lunch anf supper. mostly supper.
Just a thought. Does it happen more with a morning meal or an evening meal? Your adrenal cortisol could be too low. Your doc should test for this if the problem continues.
It probably is your blood pressure dropping. It's called postprandial hypotension- low blood pressure after eating a meal. Your blood vessels may not be constricting enough to maintain blood pressure.
Drinking a caffeinated beverage before a meal may help, and a low-carb diet, but you may want to see your doctor about this, as there are certain conditions, such as diabetes and hypertension which can lead to postprandial hypotension.