Knee pain

[Deleted User]

Hi all

I'm having problems with my left knee, which hurts whenever I run.  The pain is behind the knee cap.  It gets worse the longer I run and to a lesser extent with more speed too.  I did a 14 mile run today (long for me) and it was really bad.  However, once I stopped running, the pain subsided by 90% within 10 minutes (always does this).

Any thoughts for what I can do to fix this?  Many thanks, Nick.