I have a headache that aches a lot when I stand up or sit but not when I lie down. What could it be? I don't feel any other kind of weakness. I was feeling dehydrated some days ago so took rest and drank lots of water but still headache persists, no other weakness or symptom. Even got my eyes checked if eyesight had weakened but it's the same as earlier. The headache is all over the head, I can feel it on all sides of the skull.