Sara just beat me to it.....unless it's too far or awkward, I'd be bringing her home for the night.
Many vets just set up a slow drip IV and go home til 7 am.
Even if you have to give her a bit of water in the night, I'd be looking after her.
If the vet says there were no complications, I'd wonder why she has to stay.
I've never had an animal stay after a spay, ask if he charges for the "care" she won't be getting...it might be a boarding fee.
I say that, not to be mean but my vet charges if people wish to have extra observation of their pet.
Glad she's come through it, it's nerve wracking, isn't it??