Headache due to low blood sugar or detox?

I started this fast for several reasons but one is my body is soooo sensitive to blood Sugar fluctuations that I get alot of migraines and was hoping to heal that somewhat. I also have thyroid nodules, hair loss and am overweight by 30lbs due to the thyroid.

Anyway, this is the first day of my fast, so when I woke up this morning and didn't eat, I got this headache within a couple hours of not eating (although it had actually been 13 hrs since I last ate the night before) and it has stayed with me all day, and it's starting to get to me, although it's not pounding.

If it was a detox headache wouldn't it take some time before I got the headache? This is my first water fast..I normally do juice fasts. Oddly I'm not hungry at all, have no joint pain or tooth pain, so I'm paranoid this isn't a detox headache and won't go away.