What Causes Knee Pain?

Hi doc, I am 35yrs, women, working. Since 6 mths ++ ago I have pain in the knee and back of the knee when the leg dangle especially when traveling in the car or sitting on a chair for more than 30 minutes. only on the right foot. doctor can explain what is the likelihood of a pain. I have never fallen or had any accidents involving the legs. thanks for the explanation given