Not eating much on the day of the spay is quite normal as it hurts quite a lot and an operation is always kinda traumatizing for the bunny. How is she eating now? Her hay? I generally have a stash of special hay (the very pricey one, like second crop or Crau hay) that I use when one of my rabbit is ill / stressed / after a vet visit / when I go to a business trip... in short, everytime that something could disrupt my rabbit's diet. It encourages the rabbit to eat more fibers and help to avoid an upset tummy.
Which meds are you giving? Generally, after a spay, rabbits are given Meloxicam (metacam) for the pain but some vets also give antibiotics. Antibiotics can provoke weird kinda soft poos. If it continues for more than a day, I would give your vet a call. It might be a good idea to stop the meds (with your vet's agreement) if the spay wound looks clean and not inflammed. The meds are generally given for 5-7 days anyway.
So, it's not normal but it's probably nothing really worrying if it doesn't last for long.