Hello~I am sorry you are feeling this way. I have read and heard that after we eat a heavy meal, the body has to work harder to digest the food, so, perhaps this is what is happening to cause the light-headed feeling, the body is sending the blood to what it considers to be more important areas to help aid in the digestion. Have you discussed this with your GP?
I would try taking a good, high potency digestive aid, this will help put the friendly bacteria in your system, thus, aiding the body in digestion of food better.