Go to r/fasting and read in the sidebar re: electrolytes. Your headache is likely low sodium. Put 1/2 tsp salt in a glass of water and sip or sip some pickle juice or olive juice. Too much plain water w/o electrolytes can end up flushing out more. Hydration is a balance and sodium is key. Feel better!
Ugh, that does not sound pleasant. Are you prone to headaches even when not fasting? Do you have low blood sugar? Maybe have some coffee, I know that if I do get a headache it’s usually caffeine related. Best!