One of my poodles had a knee that went "out" on him a couple of times. It was sort of the same thing as with Buddy except there was no lump. He wouldn't put any weight on it. The vet showed me how to move it back into place, but it happened a little too often and he eventually put a pin in it and after that all was well. It was his patella joint, which I think was his kneecap. I hope Buddy will be OK. I know you will be worried until he sees the vet. Maybe he was running and twisted his leg some way. All fingers and paws are crossed that it's just something minor.