This might be an individual reaction to fasting, but I always have headaches when I don't eat properly (skip meals, etc). Is this something that I would have to get used to when eating less?
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.