What I can imagine is that you are locking your knee. Riding bareback you might be locking your knee and then using the entire leg as one solid block for balancing. Not so rare, and better than rounding your legs and gripping like a tick.
Try really relaxing your leg at the knee until your calf actually falls along the side of the horse's barrel. Stay so soft that you can feel his chest expand and contract as he breathes. Work on feeling and followoing his breathe. If you are stiff, you will not be able to feel it. Best done at a walk.