Particularly if you're also experiencing sore or cramping muscles, it could be that your potassium intake is low. If you enter your daily food intake on fitday.com , go to the reports section and check to see how much potassium you've had. The recommended intake is 1500-3500 mg, the higher end being for athletes, etc. Many low-carbers take low-dose potassium suppliments with their meals, but check with a doctor first if you have any heart problems.