What Could Be The Reason For Swelling Of Knee Post Injury ?

I injured my knee as a teenager. 14 years later I now have a hard lump about the size of a golf ball near the area I injured long ago. It is located on my right leg - to the right and below the knee cap - aroung where the collateral ligament and fibula meet. It grew recently. My knee started getting sore a few months ago when I moved into a 3rd floor apartment. It was worsened when I did a couple of 30 minutes cycling sessions at the gym. At the start if the second session it was really sore. After that i could tell it was swollen compared to my left leg, but it grew rapidly over Christmas break. I have been doing alot of walking between the airpory, shopping, and events.