The Inside Of My Right Knee Started To Hurt Suddenly

The inside of my right knee started to hurt suddenly after spending several hours hunting in a wooded area with significant elevation changes. The pain is severe if I try to raise my leg, such as putting a sock on, I am unable to bring the leg up without lifting it with my hands to bring it over my left leg. There is some swelling, no pain to the touch, and no bruising . I do not remember slipping at all today that would account for the pain. If I sit with both legs elevated the right knee appears to be off center. Any ideas as to what may be the problem?