She has been extremely active every day and since yesterday she acts "out of it" and always sleeping. She normally eats all the time and hasn't eaten all day today. Her litter box indicates she is voiding still though. I'm concerned.

Cats sleep as much as 16 hours a day. Maybe she finally "ran out of gas." Keep an eye on her for a day or two. If she's still lethargic and not eating on Tuesday, take her to the vet or at least call for advice.

Thanks!