Update - I was right to be worried about Onyx. She was just too lethargic. I took her in to the vet and poor kitty, she was running a fever of 105 degrees! She was given penicillin and antibiotics to bring that down and fight off whatever virus she's got - we have a kitten we adopted less than 2 weeks ago from a shelter so Onyx probably picked off a virus from her. Everyone always says you know your furkids best, and I'm glad I listened to my gut instincts.