Runner’s knee or, to give it its more technical name, patella femoral pain syndrome (PFPS), is typical in runners but can also occur in the more inactive members of the population.
Pain may be sharp and localised or more achy and diffuse. Sufferers often complain of pain not only when running, but when going down stairs and walking. The pain arises from the knee cap (patella) not sitting snugly in its groove on the thigh bone (femur), which results in tissue irritation and pain.
It is difficult to isolate a specific reason and the problem is often multifactorial, typically at least one or more of the below can cause PFPS.
In short, go to see a physiotherapist.
A good physio will do a biomechanical assessment to assess strength, movement control and tissue tightness in both static and dynamic postures.
Treatment includes a combination of exercises to simply stretch what’s tight, strengthen what’s weak and improve the movement control wherever it’s lacking. They may also advise you on footwear or suggest you see a podiatrist. Training will need to be modified according to how painful your knee is at the time, and this can be guided by your physio.