Retaining water weight

Okay so I woke up with my hands, feet, and face all puffy. Not really swollen but "puffy" my mom says I'm retaining water weight.

But why would I be retaining water weight??

I've tried googling it but it all seems to be over my head. Can someone dumb it down for me ?

The only medication I'm on is Depo

I do have a thyroid disorder (if that's what its called) that is untreated.