It’s a common situation that could occur to those new beginners. Caffeine withdrawal, dehydration, and low blood sugar are the most common reason.
You don’t have to stop fasting when you have a slight symptom. If it lasts for a long time or you cannot tolerate it, you need to stop and ask your doctor for medical advice.
For slight headaches, you can try to drink more water, have caffeine intake, or get a rest to see if you’re feeling better.