Just wanted to give an update...the good news is, she has started eating and drinking again, and seems to be a little less lethargic (though she has been sleeping a lot, but I think that is stress-related from being sick and the new cat in the house, etc). There was no official diagnosis though as a precaution the vet sent us home with meds and gave her a shot yesterday and ran some tests. Unfortunately, we wouldn't get the results back till tomorrow. I brought up the calcivirus, she didn't exhibit any URI symptoms but did have a suspicious bump on her nose which concerned me when I first saw it, wondered if it was an ulcer. Plus the vomiting and diarrhea. Luckily, her stools are getting a little firmer and she is urinating regularly.
I will definitely post the final results tomorrow. Thanks all!