I would guess Caffeine withdrawal is it. That's the biggest headache inducer for diet changes. If you used to drink it daily in high amounts, dropping it low will give you a couple weeks of headaches. I refuse to stop drinking caffeine for that reason largely, which my doctor says is actually a symptom of caffeine addiction but he said it has no serious issues. Just drink diet soda daily OP. Nothing wrong with that, but you should not have more than a couple of soda or coffee's worth of caffeine a day so stay off the energy drinks