What comes to my mind is mineral imbalance, usually potassium, magnesium, and/or sodium. Eating low carb is very diuretic for the body, especially when first beginning it. You can lose a lot of minerals in excretion, especially if you are not eating or supplementing to replace them. Potassium is the big offender, which causes all sorts of nuisances if too low... body aches, muscles cramps, sweats, major fatigue. Being low on Mg contributes to nervousness (ie shakiness, anxiety, sleeplessness). Supplemental magnesium can provide some relief, just not mag oxide because it's very poorly absorbed by the body. In fact, you can get a lot more magnesium into your body through your skin by soaking in a bath of epsom salts that by taking a supplement.
Excess calcium also depletes magnesium... normal adults really don't need to be taking extra calcium in supplement form if they are eating LC... LC eating prevents leaching of calcium from the body.