Possibly related: my PT has me doing some exercises to mobilize my pelvic joints (SI and pubic), which were so locked when I first saw him that he didn't believe that my massage therapist knew what he was talking about when he said I'd have some SIJ mobility at the end of a massage ...

anyway, the exercises consist of lying on my back with a wedge (i.e. a small folded towel) under my sacrum. For one exercise I pull my knees to my shoulders (knees apart, feet together), hold, release just to tap the ground and repeat. For the other exercise, still lying on my back with the sacrum wedge, knees bent, feet on the floor, I put a medium-sized ball between my knees and a yoga strap just above my knees, and do alternating abduction and adduction - the resistance means that rather than my knees moving, the femurs push and pull the halves of my pelvis while the wedge keeps the sacrum still.

Aaaaanyway, what I've noticed is that I really have to concentrate to keep my pelvis neutral, because if I tilt it even slightly to the posterior (which is where it likes to live), the wedge doesn't do its job, and I don't get the benefit of the exercise. Raising your feet is probably going to encourage your pelvis to tilt posteriorly. Beyond that I'm really not clear as to the biomechanics, whether the weight's going onto the sacrum and off the ilia or what, but you might be experiencing something similar to what I do when I do the exercises.