Her bones and joints are likely arthritic, and jumping down like that after exertion on the beach could’ve caused a searing pain, but the fact that it went away is a good sign. You might have to consider that he’s older now and try to gently lift him down from heights and things like that as he ages. I’m glad it worked out. We’ve had lots of leg pain issues with various dogs over the years. I spent a lot of money on vet appts but only rarely was there an issue. Like a pp, one of my dogs had an osteosarcoma of his front leg, and it revealed itself with pain in the leg, and over time it grew so the leg got swollen and really painful just to walk, etc. But mostly strain issues that resolved themselves with rest.