Are you taking potassium? You may be low on that since low carbing also causes you to lose a lot of fluids through urination and those minerals go right with it, especially if you are losing weight quickly. Check with your doctor or pharmacist before adding potassium if you are taking any kind of blood pressure medication or diuretics, but otherwise you can safely add up to 200-400 mg. per day as a supplement. You would also want to take it with a meal as it can irritate if taken on an empty stomach.
Hope you get that energy back soon!