What would cause knee pain to radiate to different areas of the leg?

I have chronic patellofemoral pain in my right knee from a "sliding" kneecap. I've done fairly well at managing the problem. A few days ago I was stepping down from a step ladder and felt excruciating sudden pain in my right knee accompanied by a loud cracking sound.

Since then I've had severe pain in my knee and pain radiating up the inside of my leg from my knee to my groin and pain radiating up the back of my leg from my knee to my buttocks. I have an appointment to see the doc tomorrow, but I'm really wondering what in the world might be wrong. Are these symptoms of a pinched nerve or torn cartilage or something?