I think go back and see your usual vet early next week, or at least phone him and talk through what was said today, as it's very clear from your posts that you're worried about it, as I would be too if it happened to Wills or Louis.
You could still be in the position of not having a definite diagnosis of either a sprain or hip related pain, but having said that, if Kooki continues to improve over the next couple of days and you then see your usual vet a diagnosis may be more easily made by then.
In the meantime, please take the advice of the vet seen today and keep Kooki quiet for now. Dogs don't understand that rest can help and if they are bored and want to play or run they'll forget the aches and pains in those seconds or minutes of activity but might hurt even more afterwards.
My first Cocker had his hips x-rayed when he was about 8 or so after an incident of pain that a vet thought could be hip related and the x-ray showed that one hip had a very poorly formed joint. But he went on to live until almost 17 and stayed pretty sound. So a diagnosis of dodgy hips, if that's what it turns out to be, may not be anywhere near as bad as you fear.