I always had coming-down headaches, myself. I was able to rule out low blood sugar in myself because the headaches were still there after I'd eaten. What I ended up attributing them to was some sort of vaso-spasm in my head. Just as cannabis relieves headaches in some people, it can trigger them in others. It always made my blood pressure drop pretty significantly (after an initial spike) and I expect that that same vaso-dilation was what triggered the headaches. I've read several reports of post-smoking headaches here over the last couple of years. We'll mark that down as yet another area that needs to be studied in humans: the effect of smoked cannabis on headaches, intracranial pressure and neuro-vaso physiology. I've read abstracts, some of which are among Granny's links, about the effects of isolated synthetic THC on raising intracranial pressure, but we need to study the bigger picture with actual weed.