When you drink a lot of water, then do some sort of physical activity. You are moving so much that can feel the water moving around in your stomach, and it hurts.
Suzy was playing soccer and got a drink of water. She went back into the game but couldn't run because she had water belly.
When you drink too much and you can feel the water moving around in your stomach. This use of the term water belly is in no way related to sheep-oriented diseases.
I drank a litre of water, and I have a water belly now.