Dirty water. Not the shower water but the water in the body. You're dehydrated.

Headaches, lightheadedness upon standing - or "getting up too fast" - and/or indigestion mean chronic dehydration. If you're going to say that you drink plenty of water - which people seem to say whether they do or not, I don't know why - then just drink a glass before you shower. And that's water. Not juice or tea or beer or whatever you think you're getting water from. None of that is water until the body has stolen it from the colon and cleaned it up as best it can - which isn't very well when you're dehydrated - hence, dehydration means you're forcing the body to try to do what it has to do with "dirty water".

Dirty water = dirty blood = dirty liver = "emergency mode" which means the body is creating pressure to keep the heart banging to get dirty blood to the brain. All while sacrificing "less" vital organs to keep you upright and conscious so you can escape predators but if it gets too dirty or the blood volume gets too low, it will lay you out flat. If that's what it takes to save the brain and/or heart from oxygen deprivation an/or adrenaline which is all bad news if you're being chased by a tiger.