I did have a similar problem with Willow Lymecaps, you are right. Mainly the issue became her hormonal behaviour afterwards (that was a joy a minute!!! not!) but initially Willow had a nasty infection in her wound and this made her quite unwell, she too didn't want to eat or drink.
It was pretty obvious that the wound was infected though as she had oozing, the vet had to remove the remaining stitches, clean her up and stitch her again. She was very traumatised by the whole thing and didn't eat properly for a long time afterwards.
In this instance though it doesn't appear that there is any infection (I hope the vet would have picked up on that if there was).
jdiamond, how is she other than this - is she depressed or quieter than usual? Appear to be in any pain? Going to the litter box properly?
The spey operation is a big operation and she could just be feeling generally yukky from the whole experience but obviously it's a big worry when our babies aren't eating and drinking properly.