I used to have the same problem. Probably low blood sugar if you're accustomed to a more carb rich diet. Switching to eating more healthy fats (olive oil, raw coconut oil, butter) and less sugar helped me with this particular problem.
Yeah, I'm doing salad-with-fats lunches now to hopefully aid the next attempt, and I rarely eat breakfast anyway but when I do it's already mostly fats 90% of the time (full-fat yoghurt with no added sugar + nuts).
Doubt I'd be able to go full-on keto and still eat dinner with my family, and the diet sounds pretty miserable anyway but I'm hoping this plus trying a fast again every week or so will get me past the problem.
Just want it to be known that, if you haven't tried it and don't know how you'll react, even a shortish fast may leave you feeling like you're an hour and a half in to a visit to a beer festival, which might be problematic if you have to actually do stuff that day. Maybe not doctor's-supervision-required, but consider doing the first couple attempts on days when you have no responsibilities (unfortunately much harder, IMO, since it's easier to fast when busy and not at home, but hey.)
