She might have hurt a ligament which you can read about here
http://www.marvistavet.com/html/body_ruptured_anterior_cruciate_lig.html
Sticking the leg out to pop the knee back in is common with this.
When the knee shifts its painful but it doesn't shift all the time.
Hope this helps you!
Please press Accept if you find my answer helpful. Want more info? Just reply on your question!
Bonuses for good work are always appreciated!
Thanks for the opportunity to help you!