That's very common at the start of Attack and is just your body detoxing from muck and carbs and whatever else you were having that you're not having now, e.g. Coke etc! Don't fret, drink your water and just keep on track and it will pass x
I remember when I was pregnant with my first, I craved full fat coke (no wonder I put on 6stone!!!), drank loads of it. When I suddenly switched to water because my gyno was worried about my weight I got massive migraines, I thought it was the water but soon realised it was caffine withdrawl and sugar withdrawl. Switched to diet or caffinee free and felt loads better.
I rarely drink fizzy drinks now, I try and drink more water which I find really helps with headaches.