take a look under "newbies questions" and karen's official "I feel tired" thread.

the short answers are

possibly potassium depletion--either pills or lite salt (potassium salt) will cure this (hint this is the problem: achy legs, too)

or it'll resolve itself in a couple days

raising carbs no more than 5 grams per week (every 2 weeks would be better), and mostly in the form of veggies.

HTH!