N.P.
it may be what others have said, or it may be nothing at all. If it's the overnight diaper or one that was left on for a while then the disposable diaper breaks down and the crystals that absorb the pee show up where you can see them. If this is happening when it's just a little bit wet then I would suspect dehydration but if they are soaked when it is happening then it's just the diaper breaking down.