i had the same problem earlier this year with my left leg. i went to the orthepedist and he said i might have torm a ligiment. but to be sure they wanted me to have an mri. it turns out i didnt tear a ligiment, if i did i would have needed surgery. instead, there was something wrong with the muscles above my knee. they were to weak or something. i had to do physical therapy to help it. however, there is no way to tell what is wrong until u go to the doctor.