its because food takes a fair bit of energy to digest, especially heavy stuff. oxygen goes from the brain to the stomach to assist this, hence the usual 3pm problems we all suffer !
sugar is a really crap way to get loads of energy. you are probably having something that will provide a slow release, like a banana - like this one: :banana: ;-)
carbohydrates need to be timed right - I usually have carby food about an hour before playing football, and will top up with a sport drink during the game.
keeping well hydrated is a big help too.
