My kitty had/ has luxating patella on both knees as well. I noticed that she would lay down to eat and rarely sat on her hind legs. I finally figured out that this was something to have a vet look at. When I took her in, he confirmed that she had luxating patella on both her knees; her right one...
I agree with CatLadyVetTech with Care Credit. I have one just in case at a limit of $1000! Or maybe you can look at a low-cost clinic in your area? And cats are very good at disguising pain so she might actually be in pain but not show it.