My personal thoughts are that she is loosing the muscle tone back there as she should just before she foals. In the top photo it almost looks like she is much more hollow on either side of the upside down v. This is a good indication she is going to foal soon. Again, if she were mine I would be watching very closely. Right now it is jus about 6 weeks since you noticed her bagging. She seems to be on target.
Any changes in her milk color/stickyness?