Knee pain after sleep, cold, inactivity

Hi,
During the past two years, sometimes I wake up with knee pain. Most of the time its my left knee, and some times I feel same pain in my right knee.

I go to sleep without any pain, but when I wake up in the morning or during the knight and try to move around, it feels stiff, and I cannot bend it. the only that makes it feel better, is to try to move around and walk, it hurts a lot, but after an hour or two of "warming up", it gets back to normal.

It's not swollen, and when I compare it with the other (fine) knee, they look identical, I cannot see any difference.

Background:
I had no accidents, or any physical activity that would explain this pain. the problem happens normally after I wake up, but I also noticed that when it's cold during the night, it happens more frequently. and also during inactivity (eg. when sitting for a while in the theater watching a movie, etc.). The problems happens once every week or two, and normally I recover in the same day, and sometimes it takes 2-3 to recover.

What I tried so far:
Blood test - negative
MRI scan - negative
Gout - negative

I noticed that somebody else has same problem posted in this forum: //www.curezone.org/forums/fm.asp?i=1338306


Tried the pillow/pants thing, but it didn't help.

Appreciate your feedback.
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