You don't even have to be fasting to get headaches... anything that detoxes the body can cause them. It totally depends on how rich your diet was before detox. For instance.. when I was teaching myself to eat raw veggies (I had never done this before), I ate some raw cauliflower and baby carrots. Not much at all, just a snack in place of my usual cookie binge. I woke up in the middle of the night feeling like I would die.. I couldn't turn my head, I could literally feel the blood pounding in my temples and forehead. I had to call in sick for work. I do not really believe it's a case of imagination, because I had no information on detox at this time. It was only later, when I read about it, that I realized it was the raw veggies that did it. Or, more correctly, the lack of hideous junk my body was used to!