Thanks for writing in.
From what you say, it looks like a cystic swelling around your patella
region of knee joint. In simple terms it is a fluid filled structure present since two years, slowly growing.
The commonest swellings around patella could be due to bursitis
and you need to go to an orthopedic surgeon for further guidance and treatment. He may ask you to get X rays or scans of the region.
Nothing to worry but you must see the doctor soon.
Hope this helps.