my experience is that the first 3-5 days of going low carb I run into fatigue, head ache and craving challenges. I tough it out. I take tylenol/motrin for the head ache, do not increase coffee (in fact I drink more water--flush flush) and fight through it. i look it as this way, if I was to start running 5 miles daily I'm going to be sore for the first several days. The fatigue/headache/cravings is not that bad
Keep looking ahead, be proud of yourself. Keep a good attitude. It will pass. It really will and when it does you'll have more energy than you'll know what to do with it.