What is this knee and leg pain/ache?

I'm 31 and not sporty so its not a sports injury but for a few years now I have had pain in my knee all around the knee cap worse in cold weather but bearable but a few days ago it got worse and now hurts behind the knee and my calf and shin both ache now if I stand or walk for a long time and climbing the stairs hurts too,any ideas what it could be? Saw the doctor a while ago and was just given anti-inflamatries which were no use.