What Causes Knee Pain?

In November, I worked long hours and spent most of that time on my feet, running up and down stairs. After this time, I spent alot of time sitting on the floor with my legs stretched out and a laptop on my legs. Ever since then, I have had knee pain in my right leg, at the back of the knee. It hurts to bend my leg, and sometimes to put weight on it. I can do some exercising, like walking which seems to make it feel better, but nothing where I bend it. What does the problem sound like, and what should I do for it?