I don't have a clue as to what is wrong, By no means do I know anything compared to everyone else here.
Im just curious,:whatgoat: is her back actually this long or is that just the way the picture was taken? She looks twice as long as she should be and sway backed.
I would say that something in her back end is hurting her. See how she is squatting on her back legs and they are not extended fully? That is causing her back to sink down. Have you checked her feet, legs, and stifle joints?