What Causes Knee Pain?

Hi, I'm a 34 year old woman who suffers from knee pain when I exercise. Not immediately, but after doing the activity for 5 to 10 minutes or so. In the past, I've varied my gym activity to avoid jogging. when it is always worse. Now I'm not a member of a gym and want to go out jogging. But apart from the dull ache whilst I run, my knees are incredibly hot to the touch and bright red once I've finished, even after only 20 minutes. Do you have any idea why this could be, and how I treat it? At just under 12 stone and 5ft 7 I am a bit overweight... but still had this problem when I weighed a stone less. Thank you