I have four girls, one yet to be spayed. My experience is that (after 24 hours at the vet) they come home a little tired. I suspect you are seeing what I don't normally see, because the cat is at the vets for a little over 24 hours. I have heard of vets giving pain meds right before the let them go home. I have seen swelling, but nothing that needed attention beyond keeping the incision clean and the cat inactive. I bet the vet would give you some meds if you asked.

Poor little girl.

SAS