I used to get severe knee pain all the time from bowling. At the time it had me very worried because I was relatively young and in pretty good shape. I wondered if all the sports (espescially hockey) had caught up with me.
I went to a doctor to get checked out. He couldn't figure out what it was so he sent me to a sports medicine specialist who was well known for treating knee injuries. He did an MRI on me and some simple tests in his office where I went through the bowling motion for him and did some other stretches. This is where he figured out the problem.
Apparently with the repetition of sliding/bending of the knee at the end of the approach I had over-developed the outer quad muscle. Over-developed in comparison to the inner quad. This effectively was pulling my kneecap over towards the outside which was causing me all the pain, and the need to take a lot of advil. Fortunately it was not something that needed surgery. I was enlisted in a bi-weekly physical theropy session with special excersizes designed to offset the overdeveloped muscle. I did the theropy for about 2 months and was already feeling much better.
Having learned my lesson from this situation I started to do more excersize that would help keep my legs balanced in strength so this wouldn't happen again. My favorites are the elipticle trainer, biking, and jogging. I don't do too many free weights anymore, but the leg weight program I had going for a while worked well to combat knee problems. (leg extensions, squats, leg curls, toe raises, and stretching).