Can I get dehydrated by drinking too much water?
Watch out for water toxicity and hyponatremia (low sodium due to water gain).
Water by itself is a hypotonic solution lacking salt. You can dehydrate yourself with hypotonic fluids in massive amounts despite drinking a lot of volume. Dehydration can happen even with salt/electrolyte solutions like Gatorade or Powerade or even Pedialyte.
Remember, water follows salt.
If your head is hurting after drinking continuously over several hours, your brain could be swelling. This is why they serve beer at the end of marathons- to diurese and eliminate free water.
I’ve taken care of frat guys in the emergency room due to water intoxication or milk intoxication and this can be a life/brain threatening injury if not careful.
Moderate amounts of water and electrolytes over a long period of time is probably fine with normal functioning kidneys and watching out for symptoms such as headache, lack of sweating, and general malaise.