My 19 yrs old daughter hurt her left knee when she bends her knee she has pain behind her knee and down her calf?

Knee Pain: Pain behind the knee can be from a muscle sprain and sometimes from a cyst. Has she had a specific injury ? Does she have the pain only when she flexes her knee ? I think we need a little more history to help. The main concerns are locking, catching and giving out of the knee. Those are often related to problems that may require surgery.