Dizziness that you experience may be due to a low blood pressure.
Low blood pressure, again could have a number of causes.
Like you mention, you take lots of water but you may not be taking enough electrolyte rich solutions like gatorade or pedialyte that may be the cause of your dizziness.
Another cause of dizziness could be what is called as benign positional vertigo.
This is an extremely common problem and usually will go away in a few weeks time at the most.
Meanwhile to help with the dizziness I would recommend taking something like stemetil which contains prochlorperazine.
This medication acts on the inner ear thereby reducing the dizziness.
Dizziness associated with tiredness could be due to another pretty uncommon condition called as Addison's disease and this is due to low cortisol levels in the body.
So if your symptoms persist I would recommend having your cortisol levels tested by doing a test called as synthetic ACTH stimulation test or also called as synacthen test.
I would also like that you should have your thyroid tested as a low level of thyroid hormone in the body can also lead to fatigue and dizziness among other symptoms.
Let me know if you have more questions.
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I am sorry for the delayed response as I was dealing with a few patients in my clinic here.