chocolate i do not think so, sugar high that crashes down maybe? or just you are used to have sugar input at that time and as you did not get it your body burned through its sugar and then headache, happens to me we i forget to eat for a bit too long... (well usually it happens mid 2nd day of not eating because other things to do/stress)
although i am not a doctor, if the symptoms persist got see a real doctor, not D. GeekHack (although being french it is easy to say that, i dunno how it works where you live with doctors and money)