What Causes Shredded Meniscus In Left Knee And Intense Pain In Back?

I have been diagnosed w/a shredded meniscus in my left knee after experiencing intense pain in the back of my left thigh. Physical therapy helped me a great deal and I continue to do the exercises recommended. My pain has subsided for the most part and my activity level is almost normal. However, I still experience moderate to severe pain in the back of my left thigh that is particularly painful when sitting and when I stand, it is hard to straighten my leg. My doctor keeps telling me this is symptom of arthritis in my knee, but the pain in nowhere near my knee cap. and I think it is from a different source other than my knee.