how much water are you drinking at one time?

If you drink to much water at once, it acts as a diuretic and you will pee it out along with more water from your tissues. It is better to drink smaller amounts of water more frequently and evenly throughout the day, than a lot of water all at once.
It is also important not to drink too much water with a meal and to wait about an hour and a half after eating to drink more water so that you don't disrupt your digestion.