I think your cat has been meowing to my Phoenix! Earlier in the year, he had
similar trouble, only his leg really swelled up. No injury could be found by
vet. They came to conclusion he had been stung by wasp! He had antibiotics,
painkilling jab and we got bill of about £60.
A couple of weeks ago Phoenix started limping, no swelling. Vet conclusion
after x ray was that he had ruptured the ligament over his knee joint. Xray,
pain killing jab, and antibiotics cost £163. Where they would operate with
a dog, cats usually apparently recover more or less with rest. Phoenix is
getting better, still limping, but we are told he may limp for several
months. He has recovered enough that he starts chases with his housemate
Memphis (who is twice his weight!

How he got this injury we have no idea. It usually happens through landing
badly, and is more common in heavy cats (which Phoenix is not).

I hope this is of some use to you,

Sandra