There is a condition of feeling very lightheaded when you stand: it is called orthostatic hypotension and it is not so uncommon and defined by a quick drop in blood pressure when someone stands up.
Feeling light headed when you stand may, or may not, be accompanied by your pulse rising as well.
About one in 5 women reports feeling this way, and along with this lightheaded feeling, is a possibility of falling, having other balance issues, and getting injured.
Although dehydration can lead to overall feelings of being weak, tired, lightheaded, for this specific condition of orthostatic hypotension the amount of hydration isn’t commonly the problem.
The first recommendation is to increase the SALT!
Most studies haven’t really given doctors guidance on how much extra salt, or for how long it’s safe to keep up the extra salt.
We would probably say take in about 6-10 g of sodium chloride a day. You can probably do this with adding salt to food from table salt, but you can buy salt tablets.
As a general recommendation the World Health Organization suggests you only eat 5 grams of salt a day.
Medications for orthostatic hypotension, oddly, increase the amount of supine hypotension! So avoiding medication is often what is really recommended.
And like any condition, even if you think you can figure out your own diagnosis from internet studies, see your health care provider for the most accurate diagnosis.