it may be kind of what you said, that your body is hanging on to what you gave it.
Many people are is a state of dehydration, and don't really know it. So your body confuses hunger with needing water. Hence the" if you are hungry drink a glass of water and wait 10 min".
Water is also heavy - 1 pint = to 16oz is = to 1 lb, so depending on when you drank it, when you went potty last, if you had a BM (sorry if TMI) food you ate during the day, you could have 5 lbs at the end of the day.
Give it a week of drinking more water and see what happens.
Water does a body good