Pain in knee if I straighten my left leg.

[Atri_Kundu]

Actually this began with my weird habit that I start pacing up and down my room when I have to learn something. For my exams that ended yesterday, I have prepping for a long time and that meant slow walking inside my house for hours everyday for over a month.
But from yesterday I am having sharp pain in the outside portion of my left knee cap. It is fine if the knee is at rest or folded, but everytime I try to straighten my leg or walk, I feel the pain in the knee. While walking if I keep my knee bent a little, the pain lessens.
I remember no external injury or any other strain on my legs. There is no outside pain or swelling. Also, my knee feels stiff, like the locked knuckles which you bend and crackle to make it normal.
I have tried some common medicines but none did a thing.
[Note: I am a male of 20 years.]
Don't know if I could explain my problem right.