What Caused The Black Swollen Bruise And Pain Of My Knee?

Hi, may I answer your health queries right now ? Please type your query here...I have a burning pain in my knee when I kneel, which has been going on for years. The other night I brushed my knee against the table and felt a pain similar to what a bruise might feel like. I looked at my knee and it looked puffy and swollen. It looked the same the following morning, but by that night I had dark black bruising all around the knee cap. It is sore to the touch, and if I try to kneel, but is otherwise not painful. I did not have any trauma to cause the bruising, and don't bruise easily. What could be the cause?