I Have A Diffuse Knee Pain On The Inside Of

I have a diffuse knee pain on the inside of the right knee moving both slightly behind and down to the top of the shin area (no pain around kneecap). There is no obvious swelling or specific tender spot but it becomes noticeably stiff after sitting. It came on overnight - I had not noticed any pain at all the previous day (although I did go for a run but no more exercise than usual). The next morning it was so stiff I could hardly walk. Prior to the pain I exercised regularly (3-4x week) running, football and cycling with no knee problems at all. I have now rested from running for 4weeks (but cycling is fine) - it seemed OK (ie no pain or stiffness through all everyday activities) for short jogging but I could still feel tenderness which disappeared over 1-2days each time. Then suddenly after 1 short jog it suddenly flared up and is now v painful. It stiffens after rest.