My cat was sick, is better but still wont return to the litter box.
I have 3 cats. One 13 year old Burmese female, One 11 year old Burmese female and one 4 year old rescued male. The 2 Burmese are best friends and just barely tolerate the young cat.
The 11 year old Burmese has had gastric problems her whole life and for years has defecated - mostly diarrhea on newspaper next to her litter box. She has been on antibiotics and prednizone which helps. Recently she started urinating and occasionally defecating on a portion of hardwood floor in the dining room. She is not using the litter box at all and only occasionally uses the paper.
Steroids have a number of side effects, and with a cat who has been on prednisone I would definitely consult with the vet to see what's going on.
Once you get the all clear on that, you can address the typical behavioral part of things. As you're aware, you have to clean the areas where she goes with a product designed to remove pet odor or she'll keep coming back.
Then, you can also try the usual trick of a slab of cardboard with double-sided tape on it. She probably won't walk in that area if you put that down. The other trick that I've mentioned elsewhere is to put a litter box right on the spot where she's been going.
Obviously you want to try to eliminate any sources of stress, just in case that's a contributing factor. You can also try Feliway to see if that gets her in line.
In extreme cases, as I've mentioned before, isolation retraining is sometimes the best way to get a cat back on track.
Sometimes a cat may seem as though they've recovered from an illness, but they're still not quite right so it may just take some time. With a cat that has ongoing problems, you never know what kind of stress they're feeling.
I hope that helps some and good luck with her,