Hungar,
I have a similar sort of problem but I think I know what mine is. My knees are fine until I have to start wearing my coveralls. The coveralls fit fine when I stand up, but when I sit down with my knees bent in a 90 degree angle there is some tightness to the clothing around my knees which seems to cause a stiffness and pain During gun season, when wearing my blaze orange coveralls I do not have thing problem as they are looser fitting. I haven't bought new camo coveralls, instead I change the angle of my legs and put my feet out in front of me, sometimes even hanging over the front edge of the stand. What's even better is a stand with a footrest. Hope this helps in some way.