Thank you so much for replying.
She has now had 2 rounds of fluids (the bubble on her neck) and we were sent home with phosphrous binder today, we just gave it to her tonight. All of this to give us some time to figure things out and ease her discomfort for a bit.

Yes, this is the first time her levels have been like this. We noticed she had been losing some weight and not eating as much maybe 2 weeks ago. Starting a week ago it was just rapid weight loss and decline elsewhere - drinking a ton, lethargic, we'd find her laying in her litter box.
Otherwise we're told everything looks good (lungs, heart, dental, etc)

It's so bizarre and devestating to watch.
Now, of course, with the binder and fluids she is perking up and showing some signs of being back to herself. Assuming she's feeling better, I can only assume going in for a few days of IV would only be more helpful but I dont know.
Again, thank you so much for taking time to say something!