Like anime_ted said, it’s likely her gut biome. One of the cats I took in (Moops) used to be able to kill a canary from a quarter mile away when he’d drop a load. You could always tell if it was him in the litter box no matter where you were in the house.
I think it took about 6 months to a year before the demons in his butt settled down and we could breathe easy again.
I’ve always put their meds into some wet cat food, that has usually gone over well in my case. I’ve got 4 rescue cats going in on Monday to get done and another three next month, hopefully these ones all handle things smoothly as well.