Salt is typically the cure for that, how do your electrolytes compare to the guidance in the sidebar at r/fasting?
Try sipping some unsweetened pickle or olive juice or a pinch of salt in a small glass of water (1/2 tsp or so). Try to consume more salt during your eating window.
Are you no longer drinking tea/coffee during your fast? In addition to being dehydrated, not consuming a stimulant you’re low-key addicted to is going to give you withdrawal symptoms which may include headaches.