Otto was kept overnight at the vet (their standard procedure) and he wasn’t eating well until noon the next day – and that was for a neuter.
I didn’t have Hannah when she was spayed, but her old owner told me she didn’t eat for three days and had to be forced fed. After getting her molars clipped (which i didn’t think was a big deal) she also didn’t eat for three days, so some bunnies are very sensitive to stuff like that. If it goes 24 hours I would definitely start with the Critical Care. You can take her back to the vet, but there’s not much they’ll do beyond that.
Also, red bunny urine is common and has to do with vitamins and minerals they’ve eaten. If it has blood it in you’ll see the blood spots very clearly, separate from the urine. This could be blood in the urine, or it could be that her surgery site is bleeding a bit – you might want to call the vet and ask. If it’s just an all over reddish-orange color, I think it’s probably normal.