Often the person feels a popping sensation on moving the joint. Pain
worsens on walking. It
can also be tendinitis due to inflammation of tendon at the knee
joint. You will need X-rays and MRIs of the knee to find the exact. Treatment will follow correct diagnosis. Do consult your orthopedic specialist at the earliest. Meanwhile do not bear weight on that leg, give it rest, apply ice pack, and take an anti-inflammatory pain killer. Take care!