Heremego....

That tired feeling, especially the first few days, is often the result of carb withdrawal and is very normal. Switching your body from burning glucose for energy to burning fat is a big change and you need to give your body a chance to get used to that change before you go making other big demands on it such as strenuous exercise. That's why Dr. Atkins recommends cutting back on the exercise for the first few weeks so that you body can fully adapt or just skipping it altogether until you feel more up to it.
If the tired feeling continues beyond the first week or so, it could also be a lack of calcium/magnesium or potassium due to the diuretic effect of the the induction phase. If you take prescription medication for blood pressure or diuretics, check with your doctor or pharmacist before adding supplements, but otherwise those are good minerals to add at any point.