I also experienced the same thing. This first happened in my first days of doing OMAD. I usually have headaches and stomach aches before as my body is still adjusting to the changes I have made. At first I thought eating would be the most possible solution to remove the pain but what I did is that I drank two cups of water and suddenly my headache started to diminish. From then on, I always bring a jag of water with me so that I can drink with ease whenever I will need to.
There are some headaches which come about because of dehydration. So staying hydrated is a good way to combat or prevent dehydration related headaches. If this persists however after drinking water then you'd need to try something else because then it would be obvious that the cause of the headache is different.
By the way even lemon in hot water can help relieve such headaches.