My left knee really hurt and don't know why

I had a lot of pain in my left knee joint for about three weeks but the pain disappeared over this past weekend as mysteriously as it had come.

About three weeks ago my left knee started to really hurt but only when I would climb stairs or walk up a steep walkway (it did not hurt too much going down stairs) plus it crackles and pops when I move it back and forth while sitting in a chair.

The knee was not red/inflamed or swollen at all and I could walk on it okay on level ground. The pain only occured when weight was put on the left leg to push me up to the next stair as I went through the motion of climbing stairs.

The pain was felt fairly deep directly under the left knee-cap. I'd say it was more toward the top/front of the knee then toward the back of the knee. No pain on the sides.

But like I say, now it's gone. Does anybody know what may be going on?

Thank you in advance.