I have knee issues galore. Last year in December something similar happened and my knee was doing the same thing. Like Steve was saying, lots of other locals were telling me to go to a chiro. I don't know what ART is or whatever, but the one I went obviously didn't know it. He swore that my knee issue was because my hips were out of alignment, and it was causing my back to be S shaped, which I knew was true, but I also knew that my right leg is 3/8" longer than my left. So he puts me on orthotic inserts for my shoes and wants to pop everything from my neck to my hips. In nearly no time at all, he had me to the point that I could barely walk, and it was all I could do to turn my head 1" to each side. Even when I was that stiff and in so much pain, he still wanted to forcefully pop the painful stuff, and it was so bad that I would yell out like a little girl when I was in the office. I finally just had to stop going.
On a more positive note, I saw a normal doctor and he did some X-rays and pushed my kneecap around a little. He didn't find anything torn, but, he said I have a condition, and I forget the name of it, but basically the ligaments on the outside of my kneecaps are genetically too tight. Basically if I get out of shape at all, which I was at the time, these ligaments pulling on my kneecap will cause my kneecap to not track straight, which could result in grinding and stuff if the least little thing is wrong. He told me to do leg extensions to beef up the lower inside knotty part of my thigh so that it will grab the kneecap and hold it flatter by counteracting the tight ligaments on the other side. Amazingly enough, it worked. I saw him in February I guess. By May I was going on long bike rides again. So, based on my experience, you may want to have this checked out. He showed me that a "normal" kneecap should be able to flop around all over the place pretty easily when your leg muscles are relaxed, and it's all you can do to move mine. If yours is the same way, it could be the same condition.
I don't know if it's related or not, but right now I can't hardly walk. Friday night I did 40 leg extensions in a row and woke up in the middle of the night not only sick from a stomach bug, but my knee was really sore, and has just gotten worse. I'm hoping it's not the leg extensions. Maybe I stressed it while snowboarding in Colorado last week and that was just the last straw, I'm not sure. I sure feel like crap though, limping around like an old man.