A very common running injury is Runners Knee – Iliotibial Band Syndrome. Its caused by excessive friction between your Iliotibial Band and your knee joint or hip bone when running.
This is normally because of biomechanical issues (poor running form / overpronation of the foot), that need to be addressed with the correct footwear and by analysing your running technique.
For example, some people may take larger strides when running which means they heel strike, which in turn causes medial rotation on the foot inward towards the ball of the foot, thus causing friction of the IT band on the outside of the knee against the femur.
You can address this issue by taking smaller steps when running, thus training yourself to land between your midfoot and your toes, rather than your heel. This also means you put less pressure on your joints when running, as the impact is absorbed by muscles and tendons instead.
If you think you have Runners Knee firstly you should use the RICE procedure (rest, ice, compression and elevation) until the pain subsides and you can walk/run with no pain. Taking Ibruprofen for 5-7 days after a flare up is also a good idea to bring down the inflammation.
Then when you are ready here are some ITBS Stretches to help you lengthen and strengthen the IT Band.