I'm glad you'll be taking Zoe to the vet. It's hard to diagnose symptoms over the internet...It honestly could be a number of things. Might she have gotten into something dangerous--even a plant or some kind of cleaning solution. Is she in pain from palpating anywhere. Has she gone to the bathroom.
Praying for your girl. Keep us posted about her condition please.
She doesn't appear to be in pain. Primarily she just has zero energy which is not like her at all. Also she is apprehensive with the stairs which is unusual.