When I see limping that seems to shift from one leg to the other, I always want to check for lyme disease. It may or may not be common in your area, but where there are ticks, it's a very common cause of what we call "shifting leg lameness", which is when they limp on different legs at different times. The screening test for lyme disease is simple and inexpensive. We have so much lyme where I live that many of my clients with infected dogs never actually saw a tick.
Other things that would be common, based on numbers alone, for a hind leg in a dog of Skijee's breed and size would be luxating patellas and cruciate ligament injuries.
Patellar luxations are quite common in small breed dogs - this is where the knee cap slips out of place, usually to the inside of the leg. Sometimes dogs with luxating patellas have no symptoms at all, while others can be quite lame either all the time or intermittently. Most vets are able to appreciate a luxating patella on physical exam, but sometimes if they are a grade 1-2 (meaning that the patella isn't as "loose" as the more advanced luxations), or the vet is relatively inexperienced, it might be difficult for them to tell.
Cruciate ligament ruptures tend to cause a more persistent lameness, so I'm not sure that it fits your history. The cruciate ligament in the dog is akin to the ACL in people. Dogs can strain or tear their cruciate ligament during just about any sort of exercise. It actually doesn't take as much force as most people would think, as the tear is usually preceded by weakening and degeneration of the ligament over time. An experienced vet is likely to be able to tell if the ligament is torn, however if it's only partially torn or "strained", it's not possible to tell on a physical exam. We make that diagnosis based on an educated guess. Other things that support a strained or partially torn cruciate ligament are swelling or warmth at the knee, and pain on full extension of the knee. There are also a variety of signs that can be seen on X-rays.
When I have patients who are limping but not responding well (or continuing to limp with no improvement) to medical treatment, that's when I usually request X-rays. In some stubborn cases if I can't see a problem with physical exam or X-rays, I'll refer the pet to my local surgeon for an orthopedic examination.
I hope that this helps - please let me know what questions I've brought up.
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