It could be a sprain, but with her not putting weight on a back leg and then starting to use the leg and limping, it sounds more like an ACL tear, which is not uncommon. It is hard to pinpoint the source of the pain with ACL tears considering that the pain is in the knee area and vets diagnose it with a test known as 'drawer test' which must often be done with the dog sedated as a tense dog may stabilize the leg and make diagnosis more difficult. Of course, there may be other things going on as well such as arthritis, but the ACL tear seems to fit more the picture. Have your dog seen the vet to have it sorted out. Diagnosis is not always easy with leg issues because there may be several conditions to rule out.