Thanks for the posting of your query. I had gone through the case and found that it might be osteoarthritis
and any ligament injured. You did not mention your age and other history like hypertension
or thyroid problem.
You do not worry, first go for some basic test like Thyroid profile, Calcium level, uric acid, Blood Urea, Creatnine ,RA factor, CRP, Vitamin D3
, Blood sugar level and X-ray of knee joint.
Definitely you will get the correct diagnosis. Meanwhile take Painkiller, hot water bag to knee and do not bend the knee joint. Take proper calcium rich diet.
Hope my answer will be effective for you.