It could be some kind of deficiency. What are you supplementing with? You lose a lot of water the first couple weeks on lowcarb.

Also though, I tend to get restless legs in bed when I have more physical energy than I am acting on during the day. When I'm low carb I tend to have a lot more energy, so I simply must get more exercise. If I don't, it's like there's all this energy just running around my body still, especially waist-down.