I've been dealing with inflamation under my kneecap for 35 years, and my experience has been a little different from most of the posts here. First, from your description it's not clear that you have kneecap problem. The clue that the kneecapp is inflamed is that it will be painful to walk down stairs. If you can walk down stairs without pain, it's probably a sprain of the lateral colateral ligament. Try adjusting the angle of your cleats, check that the pedal is not bent, or get pedals with more float. That may also help if the problem is an inflamed kneecap. Ice and ibuprofen are also helpful.
If it's the kneecap, one common cause is that the kneecap doesn't track properly in the groove in the femur. Sometimes (as in my case) that's caused by poor muscle development. That's why it's a common runners' injury: runners develop an imbalance between strong hamstrings and relatively weak quads. Strengthening the quads, and particularly the vastus (the part of the quad near the knee and toward the center of the body) may help. You need to be careful about strengthening the quads, because any excercise that works the quads also strains the joint that you have already injured.
Lowering your seat will put more strain on your knee, not less, and increase the range of motion as well. Lower gears will decrease the pressure on the knee, but I found higher cadences inflamed my knee more.
My solution was to work on strengthening my quads. I did squats, lunges and one leg squats. I also did short, hard interval workouts up hills in big gears on the bike once a week or less during the season. You need to warm up carefully, and do strength training only when you are completely pain free. Ice and ibuprofen after strength training is worthwhile also, even if you are not immediately in pain.
If you go to doctor, try to fnd one who treats athletes, or who is an athlete himself. Too many doctors told me to rest for a year. The best doctor told me "do anything you want as long as you can stand the pain."
I had serious deterioration to the cartilege of my kneecap that was clearly visible in Xrays. I've followed the program I descibed above for 35 years. I very occasionally get a little knee pain, but it never restricts my activity. I haven't felt any need to take glucosamine or other supplements, but I would if the problem returned and a doctor recommended it. I still ride with a local racing club, and I still coach ski racing in the winter, so I must be doing something right.