Check your sodium levels. I drink 12-15 glasses a day of water, but when I have a lot of salt, I feel heavier, until I can get rid of the water weight. By limiting your salt, the water will pass through your body in a normal fashion, and you won't have this issue.
Remember.. 8 is the MINIMUM. Water flushes out the toxins of the body. It may be another reason besides sodium, but water is a good thing.