Bad headaches are very common during fasts. I have probably ended more fasts early because of headaches than for any other reason. Sometimes the headaches are caused by caffiene withdrawal or low sugar, often they are a vascular headache (migraine) caused by an overload of toxins in the bloodstream. Drink plenty of liquids, lie down all you can, use accupressure points and EFT (see forum here for more information) for some relief. If you feel faint, see flashes of light, or blackout then take some electrolye solution quickly and consider stopping the fast. Always keep some Emergen-C or similar product with you when you are water only fasting.