Hi, I'm Dr. Bonk. I'm sorry to hear about your dog's troubles. There are a number of causes for lameness in the hind leg. The main one that comes to mind with this is a subluxating patella. This is fairly common in smaller dogs. The basic issue is that the kneecap slides out of its groove in the femur. Depending on the degree of severity it can happen infrequently or fairly regularly. This is a relatively painless issue but it does present mechanical problems and can actually 'lock' the leg in a bent position when the kneecap slides out of the groove. There is some inflammation involved that could be the reason she is reluctant to jump lately.
Another cause would be a partial or complete rupture of the ACL. This is just like in humans and causes an intermittent lameness that is more pronounced after exercise. This is more likely to occur in active dogs that are constantly jumping and twisting. The degree of the tear dictates the degree of lameness and inflammation. These dogs also tend to sit with their hind legs off to the side rather than directly underneath them.
Another cause would be a sprain or strain of the knee, hip, ankle or foot. Again, this is just like in humans. There is sometimes swelling involved and this one typically causes pain all the time, so there would be less chance of her walking normally at all.
Finally, an injury to the toes or foot pads. These are typically visible and would show pain when you manipulated them.
You can try giving her aspirin and resting her for a few days to see if that makes any difference. The dose for aspirin is 80mg (baby aspirin) per 7 lbs body weight given every 12 hours. It's best to use a buffered version if giving more than a couple doses. Strict rest means no running, jumping, leash walks only.
Both subluxating patellas and tore ACLs generally require surgery while sprains or other injuries do not. If she responds well to the aspirin and rest but the lameness returns when you stop doing this, it is time to get her to a vet for further diagnostics.
I hope this helps. Please contact me with any further questions you may have. Thank you and good luck!