It could be a strain, a pulled ligament, torn muscle, arthritis, bursitis, or tendonitis. It would be best for you to be evaulated by your physician who can run x-rays and/or an MRI to see exactly what damage has been done to your knee. From the area you are describing it sounds most likely like a torn ligament or tendon. You can try elevating it, and using some heat therapy and some Tylenol for the pain. If the pain is above the level that OTC products seem to provide relief it is best that you be seen to rule out any severe damage to your knee. I have included a link about this type of injury that I feel will be helpful to you. If you have any further questions please feel free to ask.