my dog athens as recently stopped using her back leg and when I checked where the pain was from it seemed to be the knee
Pain in the leg or knee can have many sources. Older dogs tend to get osteoarthritis regardless of the breed. Large breed dogs tend to have ruptured cranial cruciate ligaments which can occur at just about any age, but usually more on the younger side. Small breed dogs to more commonly have luxating patellas. However, all dogs can have knee pains just from traumatic injuries, just like people. There are also a number of more severe and less common reasons your dog could have pain around its knee such as bone cancer which can present in many ways.
However, it is impossible to tell what the cause is with just a description and a physical exam by a veterinarian is the best way to evaluate what is causing the pain. If the pain has been going on for more than just a few days or is so severe that your dog cannot put any weight on it, it is highly recommended you see a veterinarian and find the cause in case there is a fracture or something more serious that needs to be addressed.