Several possibilities come to mind.

First, are you taking any vitamins?? It would be a good idea. No need to buy a special "Atkins formula" (although they're fine) -- you can use any of the good-quality multiple vitamin & mineral supplements sold everywhere. Look for a formula that's "forte" or "extra", "super" or "stress" -- these will have extra B-vitamins ..

Next is potassium. Ketosis, and all the water yer drinking will flush out this mineral, so you need to take extra to keep from getting depleted. Low potassium is the most common cause of fatigue, muscle aches, etc. for lowcarbers. You can take it in pill-form, but the easiest (and cheapest) is to use a salt-substitute such as NoSalt, or HalfSalt (I think it might be called LiteSalt in the US) -- this has potassium chloride in it, and you sprinkle it on food just like regular salt. If you don't use salt on your food, one suggestion is to buy the NoSalt, which is 100% potassium chloride. Add 1/8 tsp to a glass of water (you can barely taste it .. ) which will give you 325 mg potassium.

Calcium and magnesium also play an important role in muscle function and fatigue. Consider taking a Cal-mag supplement, twice a day (take the second dose at bedtime .. ). Either 1:1 or 1:2 (ie, twice as much calcium as mag) is fine.

The final note .. do make sure you're eating enough PROTEIN. Have a small protein-y snack mid-afternoon. So often, we have our lowcarb lunch, then go all afternoon, busybusybusy, until dinnertime 6 hrs later .... and eat nothing. No wonder we feel faint! A small pouch of nuts, cheese strings, a lowcarb protein shake, a flip-top can of tuna ... Or have something ready to grab in the fridge the minute you get in the door --- some hard-boiled eggs in the shell, leftover cold meat, etc......

hope there's something helpful here.

Doreen