I have many knee problems, not only from martial arts and athletics in general but from a growth disorder as a kid that put to much strain on my bones as my muscles grew. This is called Oshgoodshlaughters <sp?> disease and is not uncommon in young athletes.
Originally Posted by MEGALEF
I can not stress enought training and walking in the proper shoe. You can go to any well trained athletic apparel store and get custom fitted for shoes that offer the proper support you need. Basicly there are three types of feet.
A nuetral (the least common) This person has a perfectly nuetral gate, or walk, and needs no support.
A Overpronater (The most common) This persons weight shifts on the ankle side of there foot as they walk or run. Although it is a slight shift it puts undue stress on your knee and hips every step you take. This can be corrected with a 'stability' shoe.
A Extreme Overpronater This person is same as above but more dramatic overpronation and needs 'motion controll' shoes.
Go check out your local running/fitness store and look into some nice shoes. I recomend Brooks, Asics, Mizuno or New Balance. I started wearing stablity shoes years ago and its done wonders for my knees.
I second going to see another doctor. Maybe not a GP, but definitely a physiotherapist (around here, they tend to be quacks, but maybe they're good in the US) and maybe even a Chiropractor, if you have a very intelligent Chiropractor. (Mine tends to do more good than physios on soft tissue, but I think mine's a lucky find.)