Yeah, headache can sometime come from lack of water! :) Dehydration. For me it can also come from noises that aren't nice, like screams and traffic sounds. Sometimes strong sunlight can make it hurt as well, but I think I'm most sensitive to really noisy sounds. It could also come from the eyes if you need to use glasses and things like that :) Maybe you have slept too little so that it comes from being tired? :)