It seems that people sometimes get headaches whilst fasting or after fasting (during the evening).
Headaches may be caused from two things.
If the body becomes too Acidic, this may cause a Headache.
If the blood sugar drops to much too rapidly, this may cause the other type of headache.
If your headache starts in the back of your neck, this type of headache is generally because the body has become too Acidic.
Now as to the other kind of headache, it is one which generally starts in behind the eyes, and this kind of headache is caused by Low blood sugars.
This kind may be solved by drinking a Lemon juice.
And some times one may have both kinds of headache as the body floats from one problem to the other.
You must identify which area the headache starts from and then apply the proper solution.
Now as to why one may get a headache whilst fasting.
When one is fasting, this causes the body to draw on it's reserves of the body, which means the body fats.
The body fats contain stored Toxins and excess body Acids, so when you release these into the Lymph system through fasting, this may cause the body to become to Acidic, thus causing your problems.