What Causes Pain Behind Knee While Bending?

I began having pain behind my knee when bending a couple of months ago. I believe this was related to exercise, I d recently added stairs (about 34 flights) to my regular walking in an attempt to get better in shape more quickly. After one of my treks up the stairs, I awoke the next morning with pain as I bent my knee to put on my socks and sneakers. Since that time, the pain has gotten worse. In addition to my knee, I had pain in my shin and especially in my foot, aggravated by driving which I do about 2 hours per day. Most recently, I m having pain in my hip as well both when driving and when sitting. I work at a computer and spend most of my day sitting. I m about 5 6 and 260 lbs, 41 years old. I have an old injury to my ankle on the same leg - a serious compound fracture of both bones near the ankle from about 20 years ago. I have arithritis pain from this old injury but the recent issue feels different, more muscle than bone related.