I can’t tell you why, but I can share some of my own experience which might help.
Most of my life I have fasted fine. It slowed me down a little, the fast, but that’s about all. However, I often felt quite sick the day *after* the fast !?!
My solution (which I fell upon by “accident” after one fast where I had to run out of the house and didn’t have much time to eat) is *not* to eat a lot right after the fast. First I drink — water. Then I drink more water….I don’t eat until I’ve drunk a few cups of water and given myself a bit of time to absorb. Then I eat lightly — usually starting with fruit and waiting a bit after that to eat anything heavier (this whole process can be an hour long — or 20 minutes. I don’t know if it matters). Even then, I try not to stuff myself. Keep drinking water between the different steps up to eating. And then drink more water say, an hour or two later (and before going to bed, and the next morning …)
Additional curious point — I suffer from migraines. One part of my treatment plan is to drink a lot of water. Nonetheless, I don’t think I have had a migraine on a fast day more than once in my life — and I’ve had migraines for 20 years!
So here are a few thoughts:
1. Maybe your body during the fast didn’t dehydrate enough to give you a headache, but after the fast, though you ate, you didn’t drink enough, and thus got a dehydration headache later?
2. Maybe there is a connection between food and headaches, so some people don’t get the headache that might be “coming to them” until ingesting some solid food?
3. Maybe some of us just have bodies that respond more slowly. This gives us a chance to head off the headaches/weakness by drinking a lot right after the fast?
In any case, whatever the cause, perhaps my solution will help you….or someone else.
Also, drinking a lot in the days before the fast can help avoid dehydration, which could help with possible headaches whether they would come during or after the fast.