Is there a way to find out why did i gain some weight after i started drinking 8 glasses of water per day?

Water is heavy: Just remember that 16oz of water = 1 lb. So drinking 16.9oz bottle gives you an instant weight gain of 16.9--this is only the physical aspect of water--its weight. You sweat, pee, poop, and breath which all eliminate water. 2/3 of body is actually water-weight. If u were dehydrated, drinking recoups this loss but gains the wt. Body tries to keep in good balance. Don't get discouraged. Good luck.