Why do you receive a headache when retaining fluids?

My mother says it is because the brain swells from hyponatremia, but doesn't that generally happen within cases where you own too low a concentration of sodium and electrolytes in the blood? I tend to retain fluids when on the pill, and seize these headaches at one and the same time (usually after eating a aromatic meal). Long-term brain swelling would probably be a bad entry, so it'd be great to know.Why do you receive a headache when retaining fluids?
You end up near extra fluid in your blood vessel and the pressure in them also puts pressure on anything surrounding them. In the brain it will explanation headaches
I'm not sure roughly the retaining fluid thing but I do know that when you put away alot of salt you bring back headaches my doctor told me to cut rear on your intake of salt and drink alot of river.