Knee Injury

[Moviefan1203]

Last night I was simply getting out a lounge chair, and I heard a loud pop come from my left knee. My wife was on the opposite side of the room and heard it with the TV on to give you an idea of how loud it was. Now I didn't have any discomfort in my knee last night, but when I got up this morning it hurt like hell. It's been hurting all day, but when I stay on my feet moving around it's not too bad. But whenever I sit down for a period of time or stand still it stiffens up and hurts a lot worse, work today was not a pleasant experience as the day went on. Now I am obviously concerned about this and have a doctors appointment scheduled for tomorrow, but I am curious if anyone on here can give me a clue as to the severity of what might be wrong with it. I'm fairly physically fit, and I don't know how getting out of a chair could cause a knee injury.