I’ve never had headaches on a prolonged fast.. have you tried Himalayan sea salt added to your water? If not I highly recommend it. It could be salt deprivation that’s causing the headache. I get lots of headaches in my non fasting life, btw. It’s the opposite for me.

No but I don't give up coffee when I fast, just black tea because I don't enjoy it without milk.

Caffeine withdrawals result in a pretty bad headache for me.

As Buddha mentioned electrolytes (salts) are required to maintain hydration. I usually take a mix of potassium chloride, magnesium chloride & table salt when fasting.

The only symptom I've had this fast has been bile coloured liquid BMs. I figure that's my body's way of eliminating some built up toxins. It might also be from the salt water which is known to have a flushing effect (although not at the amounts I'm taking).