Right knee pain while sitting

Hi,
Basically, it's like this...the longer I sit, the more it (my right knee) hurts. If I stand after sitting a long time, I just can't take it, and have to walk around for a couple minutes. I am not able to sit on the floor.

After I walk around for a couple minutes, it feels
absolutely fine. I feel as if nothing is wrong.

And if I walk around all day I'm completely fine(No Pain). I can go up
and down stairs just fine.... it's just the sitting ... if I sit too
long, that's what kills me.

Thanks in advance for any help.