This is just a theory, but i imagine because of the significant increase in calories, your gastric tract is working overtime to digest the increased food. After a meal your body redirects oxegenated blood away from areas like major muscle groups and your brain in order to get increased oxygen to the smooth muscle tissues in your digetstive tract, this leaves you feeling tired, its why a post meal siesta is always so good. I would guess that because of your increased intake, your body has to work harder to digest, hence the increased fatigue.
This obviously isnt a solution to your problem, just a bit of an explanation, But i guess you probably had a fair idea of that anyways.