Knee pain persists

9 months ago while kneeling on a hard surface in trainers with toes bent down, I kneeled back on my ankles with force as I was pulling out a tent peg that suddenly released! I felt a tight pull down the outside of my right knee accompanied with a sharp stab of pain behind the bent knee. On standing, I stretched and all seemed fine. A month later I experienced pain behind the knee when sitting for long periods, then suddenly a weakness, instabity, pain below knee and insbity to bear weight through. I was referred to physio who suggested stretches and strengthening exercises. But knee still feels odd and looks Odd compared to good knee. My GP has X-ray and says no abnormalities. Help