Hi Hayley,
The only headache i ever had during fasting was because I was a coffee drinker and didn't taper off before a 35 day master cleanse last year. I have pretty much cut caffiene out of my life now. But that was a whopper of a headache that started at about day 2 and lasted (mornings only) for about 3 days I think. Do you drink any caffiene on a regular basis?