I am going to try and explain her shape again. After checking on her, if you lok from the back, her belly looks like a w. Standing at the side, you see her natural low curve of her belly going along and then underneath that it looks like someone has stuck a tire and shes grown hair over it. So she has two curves, one bulked out that looks like a tire.
It may be a muscle problem, and she seemed to be pretty happy just now, her udder is pretty full and her parts look stretched and her tail end ready. She has dips in fromt of her hips, dips on top of her back end and dips at her back end. She is also rubbing her back end on the workshop side.
Did you ever get to see the photo of the white horse that hair of the dog posted?
Can you draw? even a cartoon version of the shape of what you see and where the 'tire' is? If you can, and could email it to me, I'd post it. Hairofthedog@freerepublic.com