Was the golden one always this way? Or did it just come on quickly? I am not sure about Arthritis but I would also check for Lymes disease. Check the dog over carefully for a tic. I know that Lymes causes joint pain and stiffness.
Definitely a situation calling for a vet exam. With that kind of lameness in such a young dog, I'd be looking at hip displaysia and resulting arthritis. He'll need an x-ray to check it out.
No Bertie seemed fine until recently. They both had kennel cough not long since. They live in Wales near the mountains and often swim in the local river. Its on a dog park and once you let them off the lead their off ( believe me I tried to stop them last time I visited, its nigh on imposssible ) They are responsible owners and Im sure she will be straight at the vet tomorrow. Thanks so much I will tell her this info