It sounds like a kneecap problem, known as Runners Knee (aka Anterior Knee Pain). It is common in active people, and also causes pain after prolonged inactivity ie sitting at the computer and going down stairs, and eases as you get going.
It is usually caused by muscle weakness and/or tightness, or can be due to anatomical features eg flat feet.
See Runners Knee for more info.
Hope that helps