I like to add there is the (rare?) condition of having drunk too much resulting in headaches. "posoined by water" as it is called in german; Or lack of salt and minerals seem the other way round.
I experienced it twice. First time I did not know about that, I thought it would be because of dehydration so I drank and drank more and of course it became worse (in like 10 min.delay)
When I checked on the inet, what could be (with my last possible focus) I saw I should eat salt. So I did eat a handful veggies with enormous amounts of salt, it became better after few minutes.
This condition is very dangerous, can lead to brain death and of course also to death!