Have Cat Questions? Ask a Cat Vet Online.
Hello. Thanks for the question.
If she is really lethargic and not eating/ drinking, then I would take her in tonight. I would first try to get her to eat some canned food and drink some water. If she'll take some food/ water, then I would worry less about dehydration. If she's lethargic and refusing food/ water, then I would worry that she is going to get dehydrated with the high volume diarrhea.
If she seems dehydrated, then I would take her in and get some fluids and antibiotics started. This should help to get these signs resolved more quickly. If she takes some food, then I'd wait it out and see your vet in the morning when they open.
I hope this helps, let me know if you have any other questions.