2. She could have stretched or ruptured her cranial cruciate ligament.
This is something humans get as well, but in humans it is called Anterior Cruciate Ligament.
THIS IS WHAT I FEEL IS MOST LIKELY FROM YOUR VIDEO!
The cranial cruciate ligament is one of the structures that stabilizes the knee joint. It is prone to problems, especially in large breed dogs. If the dog is in motion, and the lower leg is held still (by going down a hole, getting caught on a string, etc) while the upper leg keeps moving forward, there will be damage to this cranial cruciate ligament.
The symptoms seen are usually a sudden onset of hind-end lameness, with toe touching seen at standing and walking, but the dog often carries the leg when running.
Here is more information about this problem:
And you can see more here in this video: https://www.youtube.com/watch?v=PYZv1hp48J8
This is something that your vet can diagnose by checking for a "Drawer sign"when manipulating the knee. It is often corrected surgically, but if the dog has only STRETCHED the ligament and not torn it, it may heal with rest.