Just now getting caught up on this thread. So glad Grandma is better now (thank goodness for Taz!).
And I'm glad to hear there was nothing wrong with Angel's belly.
When I had Paizly "done" hers did get infected. It was obvious, greenish yellowish pussy stuff coming from the incision. I wasn't very happy about it, of course. I had her fixed at the shelter where I volunteer at a "low cost". Well it ended up costing what it probably would have cost at my own vet after the vet visit for the infection.
She also had a lump like you described on her incision. Now I can't even tell where her scar is at all. She was also my first female cat to be spayed.