Not a happy puppy

Hi, Just thought I'd let you know that over the weekend Archie hasn't been well. He's got some kind of problem with his back left leg. I haven't a clue how he's done it, or what it is. I'll be taking him to the vet to get a diagnosis. He can still walk on the leg, but when he moves he seems to be uncomfortable and in pain.
It started Saturday I think, then it was a only little problem, I initially thought he has something in his paw, but after checking it was clean. Last night something must of agitated it, so he not very comfortable at all. I’ll let you know what happens, and what damage has been done.