How long ago was her surgery? Some dogs can feel a little groggy and slightly queasy straight after surgery. My dog had her surgery early morning (spay) and was not ready to eat until about 10pm. And then she just ate a little, about half a meal. After that however, she was fine, and her appetite came back.
A couple of things to consider...what was the reason yours was spayed? Was it a routine surgery? Or as a result of a womb infection (pyometra)? If as a result of pyometra, she will no doubt still be on a course of antibiotics, and they can affect appetite sometimes. Also the infection itself can "take it out of "a dog and can sometimes put stress on the kidneys which can result if toxins in the bloodstream really get a hold. The answer to this is to catch the infection as early as possible, and IV fluid hydration.
If yours just had a routine spaying, then it does depend how long this appetite loss has been going on. Up to about 48 hours, it could well be after effects of the anesthetic. Much longer than this, there could be some cause for concern.
I would not think twice about calling your vet, or the vet who did the surgery, to ask advice. Also, if you are able, take her rectal temperature. If it is up over -say- 102.5...103 then she definitely needs to see the vet again. If you can't do this yourself, take her in anyway, and get the vet to do it.
Sometimes this "after care" is included in the surgery fee.
Anyway, if all is well, and there doesn't appear to be much wrong, and her wound is healing nicely etc.....then her appetite may just need "kick-starting" again. A few tasty treats may do the trick. If she normally gets dry food, then try her with some boiled chicken breast or fish. I made a kind of fish soup (with de-boned white fish, a little potato, and a few green beans) for my dog after surgery when she wasn't the slightest bit interested in dry kibble. Well, she sniffed at it, then next minute she was wolfing it down! And after that, there was no problem getting her to eat her normal food. This also helped get some much-needed fluids down her as because she wasn't eating she wasn't interested in drinking either.
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