The bone just below the knee cap is called the tibia. The area of the tibia just below the knee cap is called the tibial tuberosity. This is where the patellar tendon attaches to the tibia. This attachment can occasionally get inflamed and very painful. It sounds like you may have some inflammation and fluid near this connection. This type of inflammation can be quite painful and can even make walking are standing up more difficult.
Inflammation and swelling in this area is actually quite common especially in young people that are going through their adolescent growth spurt. When this occurs we call this Osgood-Schlatter disease. This disease is caused by a weak attachment of the patellar tendon to the tibia causing partial separation between the two structures. Most people grow out of the condition over time. However, it can be severe enough to make running and jumping more difficult and painful.
I suggest that you schedule an appointment with your primary care physician
. He or she can carefully examine your knee and determine what area of the knee is effected. Your doctor
may decide to order some imaging to determine what is causing this pain and swelling. I hope you feel better.