Hello, Thank you for your query. *did you sustain any injury? *did you lift any heavy weights or suddenly twisted your upper body suddenly? *what is your height and weight? *do you smoke or consume alcohol? *do you have any other medical issues or take any medications? *how old are you? You could have a ligament tear or contusion. I recommend : -an orthopedic consult -x rays of the affected knee, an mri of the knee joint may be needed. -blood tests (complete blood count, calcium levels, kidney function tests) Based in these we can arrive at a diagnosis. In the meantime, apply ice pack several times a day for about 15 minutes each time. This may help with the pain due to inflammation. Avoid any activity that worsens the pain (over extension, over flexing, standing too long, jerky movements. Wear supportive shoes. Hope this helps. Please let me know if there is anything else I can help you with.wish you good health