Iam a sports therapist and the most common reason for the left knee hurting in sports such as golf, tennis, baseball, hockey, bowling etc..is because that you put so much torque on your left leg and vice versa if you are left handed in that case your right leg. Now especially in golf your left foot is to remain stationary when the rest of your body is turning against it, naturally your foot wants to move along with the rest of your body because that is the natural thing to do; unfortunately you can't do that in golf without maintaining your form and posture. When you swing all of that velocity and inertia is being stopped by your planted left foot and leg (which is alot of velocity and inertia if you are a fast swinger) which cause stress on your leg, ankle and knee. The best thing to do is to stretch your knee out thoroughly before and after hitting balls. Also if you are addressing the ball with your feet very square and parallel to each other (pointing straight forward) i would reccommend turning your left foot out a little bit more than what you are playing at right now to avoid some of that resistance and tension on your swing follow through.

Chances are if the pain is coming from your knee then it is not a tendon, unless it it coming from the major tendons from behind your knee. It could be a number of things including bone grinding, cartilidge problems ( which is not uncommon for lifetime athletes ) , or fluid build up in the knee. If stretching and turning your foot out a little bit do not work over the next couple of weeks i would seek a physician before you end up injuring yourself further or worst permanently.

As for shoes as long as the fit comfortably and have good arches in them they shouldn't be a major factor. Because if you are a "flat footed" individual and are playing with low arched or no arched shoes that can cause all sorts of mayhem from hurting knees to a painful lower and uppr back. Stand up bare foot on a non carpet surface, if you have a good arch the peak of your arch should be approx. 1 inch from the ground. Anything less than 1/2" or if your arch is non existent and touching the surface then i would highly recommend going to get custom made sports arches for your shoes. Cost anywhere from 75.00-200.00 depending on where you go and will save you thousands years from now in preventing your from getting something called Plantar Fasciitis, then you can kiss all sports including walking pretty much good bye.

Ive seen some people in ungodly great shape with some of the worst knee problems you could imagine from sports. Especially if you are 6'2 or taller. Hope this helps you out.