If you start feeling REALLY weak and shaky, your blood sugar may be getting low. It should normalize after a few days on the diet. You body may be used to the roller-coastering blood sugar levels caused by carbs. You may need to eat more protien (since it is turned into bloodsugar, just a whole lot slower than carbs are) to help get you through the first couple of days. Hang in there. You just have to make it a few days and you should starting feeling a LOT better.