Maybe this is somehow getting mixed up with that "Don't swim immediately after eating" saying? The logic behind that one being your body is using too much blood to digest your food, and so could compromise your ability to remain swimming and thus you could accidently drown yourself in the process? Maybe your mother heard that one, didn't realize the reason behind, and so thought that getting into water after eating could be unhealthy?
There is no way that bathing right after eating could be the cause of you storing bodyfat in your belly, so don't worry about it
