I am so sorry that Pixie (and you) are going through this!
The constant panting, even in her sleep, would concern me because this could be an indication of pain. Did the vet check her teeth? Someone else mentioned anal glands....I was wondering about this, too. I have one furkid that acts completely unlike herself....like night and day.....when she's not on a higher protein diet.
I am sending positive thoughts your way. I'm glad that your vet is able to see her tomorrow. Please keep us posted.
Hugs~