After Sitting Down For A While My Knees Get Really Sore What Is The Cause Of My Lateral And Posterior Knee Pain?

What is the cause of my lateral and posterior knee pain? - after sitting down for a while my knees get really sore

I began to feel some pain in my knee a week ago. Shortly after I left the long run, I have a feeling some tightness and sharp pain on the outside of the left knee. My leg was stiff and painful for the rest of the day and it hurts very badly to go down the stairs. When I awoke the next morning I felt a little better, so I decided to go into another profession after school. About a mile, which goes back to hurt to walk but always think that the pain dissipates. Could I end my career in the day, but I had great pain in the last 3 days. I tried to run a little bit today (3 days), but the pain was too extreme, more than about 30 steps. Not definantly joint pain, pain in the tendons and ligaments and muscles in my knee and leg. I have had no pain related to the knee.

The symptoms that I experienced:
1. Acute pain outside of the knee when climbing stairs
2. Acute outside and behind the knee when walking or running too far
3. Acute pain in Latin America and the CaribbeanK from the knee to the leg stretch while sitting
4. Slight swelling on the outside of the knee

Note: I had surgery on the same stretch knee 4 years ago and again last year on its feet. I also have flat feet, but were, in the same shoes she wore when skiing off-piste.

Any help you can give me would be much appreciated!