If you're only in your first week, your body is still transitioning between fuel systems. In the first few weeks you're not yet "fat adapted". Your body is still seeking to burn glucose. Once you become fat adapted, your body will use fat as a fuel more efficiently.
Also, you my be lacking some electrolytes. (Sodium, Potassium, Magnesium, etc.) This can lead to cramps, headaches, and fatigue.
You may also want to eat more fat, as long as you're not having to force yourself to do so.