Could be hypoglycemia, which has been linked to headaches.

One thing I've observed from going on / off IF several times is that trying to jump straight into the full fasting window can cause problems. If you normally eat on a 12/12 schedule, for instance, and try to switch straight to 16/8, your body will not appreciate it.

Try very gradually building up to the window -- add another 10 or 15 minutes between meals each day.

I got used to it by doing 12-12 then 14-10 then 16-8 then 18-6. I believe I did 10 days on each phase as 1 week seemed too "quick". It was just my moderate way of weaning myself off breakfast. I have tried the 24 hour fasts and I just can't make it and not be miserable.