[telchior]

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.