Thanks for writing in. Pain
in the knee
cap usually refers to injury or under development of patella. Conditions causing this could be patellar bursitis, chodromalacia, patellar and quadriceps tendon rupture, fracture of the patella, subluxation and dislocation of patella.
In your case, it is injury due to over use. It could most likely be pre-patellar bursitis, which causes pain during movement, limited movement and swelling.