What could it be? The knee is not swollen; it looks fine. It's hard for me to describe the pain, but I guess I could say it feels a little like burning, and it's not on the surface of the knee, but inside, sometimes below the cup.
It doesn't hurt to press on the knee at all. Sit-ups are usually fine, too. When I'm sitting down, I can sometimes inflict the pain by straightening and bending my knee slowly, trying to "catch" it.
It's been 4 years and I've been to numerous doctors, yet the problem persists... If you had anything similar, I'll be glad to hear any ideas.