With Cushings they have a tremendous thirst, so I don't know what to think about your girl not drinking. My foster has mild Cushings, no medication needed right now, and she drinks her share of water, but have noticed lately she has not been drinking very much. Maybe it's because of her excellent diet I have her on.
Just leave her water down for her 24-7. I'm sure she's just upset right now. Give her another day.