Question: Is it possible for spaying a female cat to fail?
I have a 9 month old girl who is getting very heavy, specifically in her belly. She was spayed before I adopted her at three months from a shelter. She is showing signs of being pregnant, but I don't think it is possible. She is half the size of her neutered house mates but outweighs them all. We do have a stray cat that is not neutered that is hanging around my home.
Is there anyway the surgery did not work? I feel silly calling my vet to ask.
Yes, it's possible for a certain amount of uterine tissue to be left behind. However, to my knowledge, the usual result is that it only can cause estrus symptoms, which can be uncomfortable to cats. I can't imagine that she could become pregnant, but then, I'm not a veterinarian.
There are other potential causes for "heaviness in the belly," and I'd strongly urge you to have your vet take a look at her. Don't feel embarrassed at all about asking your vet. I once asked my vet if he was "sure" he had completely neutered my Jaspurr, as his testicles appear much larger than those of all my other neutered cats, past and present.
It's also possible, of course, that the shelter somehow got its records mixed up and she wasn't spayed at all. Since your email has gotten me curious now, please let me know the results of your vet's exam of your girl.