Hello from JustAnswer.
There is not a joint between the knee cap and the shin. Of course, there is the knee joint behind the knee cap, but there is no joint below the knee cap and above the shin. There also is a joint where the two bones in the lower leg are attached,which is located behind and slightly below the knee, but this joint has minimal movement, so would not be recognized as a joint by most people.
There are a couple bursae in this area that can become inflamed. There is a bursa in front of and immediately below the knee cap. There is another bursa a couple inches below the knee cap.
So, if you are feeling pain below the knee cap and not yet in the shin, it would most likely be due to inflammation of a bursa, also called a bursitis, and more likely the bursa a couple inches below the knee cap, which would be called pes anserine bursitis.
For a bursitis, it would be appropriate to take an over the counter anti-inflammatory medicine, such as ibuprofen or naproxen. It also can help to ease inflammation to apply moist heat to the area.
If I can provide any further information, please let me know.