This happens to me ALL THE TIME. As well as my right knee starts getting sore. But I'm short (5'1) so I have my stirrups all the way up. But I never noticed if I brace the stirrups or not...I'll keep that in mind.
I have a condition in my hips that makes one leg a half inch shorter, and my nerve gets pinched alot. But my knee also hurt me when riding. My saddle wasnt broken in yet, and so I wet the stirrup and used a broomstick to turn them out, and my knee didnt hurt after the next ride! It may be like others said pushing the stirrup, but if all those are a negative, maybe its not ur equipment.