This dog has a lameness problem, indicated by the fact that she has a grade 3 out of 4 lameness (only occasionally weight bearing) on her right hind leg.  However, this is complicated by the fact that she has pain in the left hind -- and in the video we see her bearing weight on her right hind leg with the left hind held off the floor.  Physical exam revealed problems with both rear legs.  The pain observed in the left hind leg could be due to unusual stresses on that joint from compensation for right hind lameness.  

We're going to focus on the right leg for the moment (since that's the leg the owner has consistently observed trouble with). There was no cranial drawer sign in the right stifle which allows us rule out cranial cruciate ligament rupture.  The medial buttressing on the right stifle is suggestive of joint instability, which could be due damage to the joint capsule.  This sort of damage could be due to trauma (falling off the bed or rough play with her housemate), an inflammatory response (could be infectious or automimmune), or a congenital abnormality.  Aspects of this case that are supportive of trauma as a cause are that the lameness was observed after she fell off the bed and that she then became clinically normal within a  month (improvement with time for healing).  The intermittent lameness could be due to continued damage to an unstable joint (an unstable joint is suggested by medial buttressing) - each time something (ligament, joint capsule) tears, there's period of lameness followed by resolution as it repairs.  There's ample opportunity for continued damage, because the household has a second dog with whom she plays.  We thought that evidence in support of an inflammatory process included the fact that she has allergies (systemic inflammation?) and/or that she has moderate tartar buildup which could lead to sepsis and septic arthritis.  Evidence against an inflammatory process is that the lameness did not appear to respond to anti-inflammatory injection.  A congenital malformation of the joint elements (trochlear ridge, ligaments, cartilage) could be the cause of this problem.  A congenitally shallow trochlear ridge could predispose this dog to patellar luxation,  causing irritation and fibrosis of the joint capsule (medial buttressing).