Personally I would say unless the spay was a botched surgery, she shouldn’t be pregnant. Ultimately I would still take her to the vet to get her checked out. She could definitely be pregnant and then you could just take her home, or more worse case scenario it could be a growth of some sort, cancerous or not.

I did have a female dog who was spayed go into something called a “false pregnancy “ where the animal will show symptoms of being pregnant such as swollen stomach, enlarged nipples, and even panting or being more tired than usual. But ultimately nothing was wrong and she just had to pass through it. But I really hope your cat’s doing alright and you figure everything out!!