I've noticed this happening to me sometimes as well. One thing that I did realize was while taking Mod, my appetite for water decreased tremendously. I normally don't drink much water to begin with, and this didn't help much. I noticed occasional headaches, but soon realized that it was all stemming from dehydration. I started to take advil and slept it off. But later after I realized that it might be dehydration, I just chugged as much water as I could and laid down to take a 15 minute nap. It's amazing what re-hydrating yourself can do for your headache. Similar to having a hangover I suppose.