A dehydration-related headache happens when your body doesn’t get enough fluids. Even mild dehydration can cause a headache. You may feel pain all over your head or in just one spot, such as the back, front or side. The pain is usually like a dull ache, but it can also be sharp. You may have a throbbing headache, or the pain might be constant. The pain might get worse when you bend over, shake your head or move around. They often get better with at-home remedies like drinking water, resting and taking over-the-counter pain relievers.