Hello.

This could be something relatively minor like a muscle sprain/ strain. I'm more worried that it's more serious as it's been going on for a week- that being an ACL tear.

The most common cause of rear limb lameness in medium to large breed dogs is ACL injury in the knee. The ACL is the stabilizing ligament in the knee. ACL tears cause pain initially then instability leading to intermittent to chronic lameness or non-weight bearing.
 
Diagnosis is made on physical exam by palpating the instability or "drawer" in the knee. Radiographs (x-rays) also help as there will be joint swelling, bony deposit build up and arthritic changes associated with the joint.
 
Treatment is surgical. There are two common procedures:
1. Lateral Suture Technique- a large fishing line type of suture is placed around the joint to stabilize the knee. This is less expensive and a less invasive procedures. There is potential break down of the suture as a complication but it's uncommon.
 
2. TPLO- this is a more recently available procedure where the shin bone is cut and rotated and then a bone plate attached to change the overall dynamics of the knee joint and stabilize. It is a more invasive procedure and more expensive. The benefit is that these guys will start using the leg a little sooner.
 
Either procedure offers near 100% recovery in 4-6 months. Either procedure will lead to more arthritis of the joint. Without surgery, it will be a source of chronic pain and instability. Keep in mind that ACL injury is a degenerative disease in dogs rather than acute injury as it is in people so many dogs (30-40%) will eventually tear the ACL on the other knee as well and need surgery on that in the future.

I hope this helps, let me know if you have any other questions.