Is this normal or should we be worried?

Normal

After being put out with anesthesia and having a procedure like being spayed, it may take quite a few days for things to get moving again internally and for her to have a bowel movement. After my male cat was neutered (which is a lot less evasive procedure), he didn't poop for maybe 4/5 days after the surgery which I was told was normal since I, like you, was concerned. He also strained/struggled the first time he went after his procedure.. which is also normal.

Just give her time.. she will eat more as the days go by and drink more..

if she doesn't have a bowel movement by Tuesday, I'd call the vet.

You didn't mention the age of your cat. My kittens gets spayed/neutered at age 2 months. They come out of the cage wanting food and to play. My cats will stagger out of the cage and just want to sleep. If they don't eat within 10 hours I will start force feeding them.

I make sure all my adult cats pee within 12 hours. And poo within 24 hours. If the cat doesn't eat much for the first 12 hours, then I would let the 24 hour rule slide.

Check the cat. If she is playing and walking normal, give her a little slack. If she is vomiting, not eating call the vet.