That is one of the frequent signs of a meniscal tear. Mine was like clicking every time I went up stairs, and I could manually "pop it out of place". Sometimes it was painful, sometimes not. Then, it would lock in place, and that was painful to wriggle my knee back to normal.
I would guess that it could be a meniscus or a cruciate injury. I'd go see a doctor. Those types of things can lead to other injuries, but most often lead to premature arthritis.
