Constant severe knee pains in left knee, why?

In my left knee, if I am in a sitting position for long periods, hurts so badly. When I stretch my leg out slowly to relieve the pain, it gets so bad I literally get paralysed in pain for about 1-5 minutes before it goes away. I am a healthy 20 year old. Been having this issue since I can remember. It may be... show more In my left knee, if I am in a sitting position for long periods, hurts so badly. When I stretch my leg out slowly to relieve the pain, it gets so bad I literally get paralysed in pain for about 1-5 minutes before it goes away.

I am a healthy 20 year old. Been having this issue since I can remember. It may be contributed to when I was younger, I had to kneel in corners for hours [generally for 12 hrs] as a form of punishment [For those who don't know: It is where you face a corner of a room and kneel down. You stay there as a punishment].

Thanks.
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