You might be dehydrated. I know that sounds stupid, but depending on your hydration levels going into the DF and your amount of water loss the last few days depending on heat and physical activity and other factors, you might be at a point where drinking some water might be all you need. What does the headache feel like?
Does it sit in one specific spot in your head and pound with every beat of your heart? Is a dull ache somewhere? Is it all over your head or only on one side or even one quadrant?