In all probability you have injured the menisci in the knee—either the
medial or the lateral depending on the site of
pain. Meniscii if torn or injured, result
in joint instability—that is you are not confident the knee joint will take your weight. Often the person feels a popping sensation on moving the joint. It can also be tendinitis due to inflammation of tendon at the knee joint. Since you had tendon injury 4 years back, this time too it could be a tendon troubling you.