What Causes Mild Pain And Pressure On Left Side Of The Knee?

Hi i m having a bit of pain at the right side and just below of my left knee cap, it isn t a bad pain more like pressure pain and I don t get it all the time but I ve had it for years and it s more often know than it used to be sometimes I get it while walking sometimes it s when i m sat down for a while after I ve been for a walk and sometimes I can get the next day if I walked a bit more than normal sometimes I find deep heat helps but I can t stop thinking that there s a bit more to this problem I ve been having