What is your diet like when you come out of fasting? Are you refeeding for a few days and restoring electrolytes before you go for a heavy meal?
I've found that there's a sweet spot as to fasting length: too soon (say a day) and I feel really hungry, so calorie count probably doesn't add up as I eat more than I 'saved'. Wait too long and the something similar happens (starving), my ideal spot seems to be ~3 days and breaking the fast with a 'regular'/not-eating-out meal.
I don't supplement for the fasting, from what I've read ~3 days isn't long enough to warrant that. Just lots of water and some salt (iodized) if I'm doing 4 days. Usually headaches may show up at the end of the 3rd day but that just tells me I'm not hydrating properly (also piss color helps get feedback on this).