Head Ache When Hungry

Sounds like a case of hypoglycemia - [low bloodsugar resulting from excessive releaseofinsulin into the bloodstream].
It's typicaly characterised by headaches, weakness and light-headedness. Your attention span might also reduce and you will get easily irritated or annoyed, etc.You willalso have a craving for sugar.

check out this link:
http://en.wikipedia.org/wiki/Reactive_hypoglycemia