20ltrs is a staggering amount per day to drink. 2 ltrs a day is what to aim for. I agree you need to see a doctor. Drinking that much will wash out all the vits and mins and possi bacteria in your body.20 litres is loads of water (recommended is 1.2). Are you sure you are drinking that amount as I thought it would be impossible, and would lead to serious health effects
http://en.wikipedia.org/wiki/Water_intoxication