A UTI can have many symptoms. Frequent urination, and paid so they tend to avoid the litter box, as they associate the pain with the box.

Straining would not be a sign of a UTI, that is more a sign of a blockage.

Also think about what food you are using. Just because she ate it before, that does not mean she CAN eat it now. She has had teeth removed and this is not the first time, so her abilities may have changed. With a slight kidney issue, a canned food in a gel is better than one in a gravy. The gel seems to help them hold fluid in better. A high quality grain free food would be best as it will put less strain on her entire system. Look for canned foods by Weruva, Fussie Cats, or something like that. Don't waste your money on grocery store brands, those are just junk foods.