I'm not a doctor but I would say getting her back to the vet is the most important thing right now. If you are waiting for an appointment with your regular vet get Cleo to an ER vet clinic. Just FYI, purring does not necessarily mean they are ok, cats also purr when stressed. I had a similar situation with one of my cats, GB, just a few weeks ago, matching most of the symptoms you listed. I noticed that he had thrown up his meal, and while not hiding, staying away from the family. GB is a 7 year old tabby that sleeps with me in my bed and the next day he would not get out of the bed. I knew something was wrong because breakfast is his favorite meal. He wouldn't eat, wouldn't drink and seemed lethargic. My regular vet was booked and unable to see him. so I took him to an ER vet clinic. They asked to keep him for observation. The next day I was told he had developed a fever overnight that spiked to just over 105. They treated him with anti-viral meds and IVs to keep him hydrated. He is now recovered and back to his old self. The ER clinic said it was a good thing I brought him in when I did because a combination of fever and dehydration can very quickly lead to organ failure. You didn't mention a fever on the first visit but then mine didn't develop a fever until that night at the ER.