It definitely is the chocolate that causes my headaches. I have found out that I can eat one cube of the biscuit cake I made and not end up with a headache. Any more and I wake up with a terrific headache the next morning. Must be the cocoa I used because I have not noticed this before with any other chocolate but I will definitely try to analyse what I have eaten the day before when I wake up again with a headache!
It is a nuisance, though, although, on the other side, it will keep my calorie intake down.