You've reminded me I did something very similar a few years ago. I bent over and twisted and felt something tear in my knee. I was in absolute agony like you but it did heal without any intervention (I think it took a few months to heal completely). I've since found out that the bending and twisting motion can cause a very common knee injury.
Hopefully whatever's happened to your knee will heal on its own Siobhan, though you should definitely get all the tests done you can to rule out anything that needs treatment.
No he was only in for a few hours but he had to have another general anaesthetic, which I was very unhappy about. He's also got very bad separation anxiety (I'm his third owner because the others couldn't cope with him) so it was a major trauma for him. Hopefully we won't have to step foot in the vet's again for a few years!
I hope it wasn't too chaotic for you!