LOL!! Renee - my girls are just the same!! Out in the field it is "What DO you think you are doing? Clear off!" While once in the stables at night ready for foaling, they take no notice of a gentle WARMED hand feeling their bags.
As Renee said, as mares get pretty close to foaling they usually 'soften' on their hindquarters either side of their spines and round where the tail bone joins the spine. Some mares really 'drop away' here and look awful - skinny like - which is why I like my girls to be well up to weight before they foal! I can still 'feel' the soft spongy area, but I cant actually SEE it. It is often quite difficult to get the mare hind end back up to scratch after foaling if they are allowed to 'drop away' too much (difficult to explain, but some folks think it is quite normal for a mare to look 'skinny' over her rear end as she gets close to foaling, whereas they should take warning that their mare needs more to eat - there is a load of difference between a normal softening and actually losing weight!)
Which is why I like to see brood mares with a good weight on the whole of their toplines and get very upset (and more than a little angry) when I hear people say "well she is heavily in foal so she will be looking a bit pulled down" Grrrrrrrr!!