This is called the low-carb flu, and yes, it's your body kicking the sugar and carb habit. You must ride it out and resist cravings even though you're going to be in really nasty shape for a few days to a week. Ride it out! Think about the fact that most carbs we eat (the grain-based ones) contain opiods, which make them as addictive as heroin. It takes your body a little bit of time to shift over to using fat for fuel instead of sugar, so while it's working that out, resist the carbs.
The best way to make it easier is to get lots of rest, and any time you feel a carb craving, eat something fatty, like bacon or tuna.
You might want to read up on how bad grains and other carbs are for your body and what they do to you. Read "The Vegetarian Myth" for some really eye-opening information.
Best of luck! Keep posting here and keeping us updated on your progress.