I have suddenly acquired pain in the back of my knee. The pain doesn t travel down my leg or extend from the upper part of my leg. I had no physical activity that caused it. I was sitting in class and went to leave class and I had the pain. I rested it last night, to see if it was just something that needed down time. The pain persisted today. I do not typically have pain when I m walking, only after climbing stairs or putting weight on the front and ball of my foot. The pain is only in my left leg, the right is unaffected. The pain is a sharp weakening pain.