You will feel muscle fatigue and weak if you aren't eating enough. On the first two weeks don't worry about how many calories you are consuming. Just watch the carb intake.
I found myself ready to fall flat on my face within 3 or 4 days of starting the diet. Now I have a ton of energy.

Make sure you are drinking lots of water and taking some type of multi vitamin/mineral supplement as well. Potasium/mag especially for muscle cramps and fatigue.