Cavalier King Charles Spaniel sudden leg injury

[HelgaHufflepuff76]

My 5 year old Cavalier just ran really fast into the garden to bark at a cat magpie on the fence. He came to an abrupt halt and yelped loudly. He came back into the house dragging his right hind leg slightly. He seems slightly better now, but is still hobbling a bit.
Obviously if it doesn't improve he'll be going to the vets, but can anyone suggest what he might have done? This has never happened before and he's my first dog.
Thanks

Many possible injuries without examination impossible to say.

My Cavalier had a similar thing when he was young, and it turned out to be a luxated patella. He had to have it pinned - it was a straightforward operation and once done, it never troubled him again.

Was Luxating patella here too (Labrador). First time knee cap popped out was similar, suddenly lots of yelping until it popped back in then a limp for a while, until next time, then started happening daily until he had an op to fix.

Is he putting weight on it yet?