It is so strange of your situation. Your eyes may get high pressure after you eat. That is why they get heavy. You need to go to the regular hospital to have a full check on the eyes. If there is any eye disease, you could treat it as soon as possible. You'd better keep healthy of your eyes and make your eyes get moisture and comfortable through the good rest and healthy diet.
In some degrees, it is normal for us to feel sleepy after eating due to the blood will relocate to our stomach, which is used to digest food. Besides, low blood-sugars always make us feel fatigue, or cause more serious conditions like weakness or even shock. So, a balanced diet are actually needed to relieve such situation. In general, when we eat more than we have been doing, we can accumulate the energy and then avoid low blood pressure. Furthermore, keep it in mind that dehydration can lead to lethargy. You need to guarantee a certain amount of liquid intake. On safe side, you can try to ask for a blood glucose levels check up from your doctor.