Looks like it was not a virus after all. I had to leave Onyx at the vet for the day, they ran her blood work, did some xrays. They were beginning to rule out a virus when the penicillin was not kicking in when it should have. They saw that her pancreas is very inflammed so the barium (used for the xrays) should help soothe that and she got more powerful antibiotics and penicillin to keep her fever down. So it appears like it is pancreatis, and I guess it can have a quick onset! She's home now and hopefully we'll see improvement quick.

Thanks for all your thoughts and prayers!
ANdrea