Over the last couple of months i have developed a pain in my left knee, however this pain only starts to kick in when i am sat down and my leg is bent. After 30 mins or so is when it starts and if i am sat down for long periods it gets alot worse. As soon as i straighten my leg out again the pain goes away after a few seconds, So driving can become difficult. It doesnt hurt when i run and only aches sometimes after playing football. Anyone got any ideas what this is?