I can not draw a picture with words myself, so I understand the problem. As a mare gets ready to foal, the baby moves from sticking out on the sides or the bottom of the sides to a position where the belly points to the ground so to speak. The baby has turned and instead of laying comfortably (for it), it then has it's front feet and nose in the birth canal. It's rump is that bulge hanging down.
That is what I believe is happening.
she had that happen, changed shape and went to a belly pointing down. Now she has this kind of flat rounded "tire" shape underneath. Its like the legs are right at the bottom wrapped round her sides? So difficult to describe. I had hoped dj would do better!