You might want your doctor to give you an A1C blood test for diabetes. Lethargy after eating has to do with diabetes/hypoglycemia. I have type 2 diabetes and in recent months my eyes are also getting blurry soon after eating, usually from salt. Diabetes effects the blood vessels including those in the eyes, and over time can be very damaging to your vision. You should also have an ophthalmologist check the blood vessels in your eyes (they do it with a kind of magnifying glass).
I have the same experience I have seen numerous doctors and no one can figure it out, I have yet to consult the g.I. Doctor though-I believe it has something to do with that and the diversion of blood to the digestive tract and away from the essential organs because I also get lethargic. Did you hear back yet about any possible cause?
I have the same symptoms (blurry vision, lethargic after eating) and came to the same conclusion (blood diverted to digestion). My Dr., likewise, had never heard of such a condition.
It could be several things wrong, you should see a Doctor. It maybe something simple, you may be eating too much salt or you may need glasses. If you are on Meds you may need to have it changed. Whatever it is you need to seek professional help soon. Take care.