have you bumped it (hard) or fallen on it? i don't just mean recently... i've fallen repeatedly (biking, hiking, etc etc) on my left knee, and now have some built up scar tissue that causes knee *cap* pain so acute that just the rub of long pants on "bad" days makes it difficult to walk (riding is usually okay, though).

or... a runner-friend of mine suggested this to me as the cause of my knee problems... could you have a torn meniscus (sp?)? visiting an orthopedic surgeon for an MRI might help determine the problem too...