When is the last time she had a bowel movement? And when you say that she stopped using her back legs, is she dragging them around behind her, or are they flopping around? Any twitches in them? Do you see any humps or curves in her spine? Is she straining to poop a lot? Any
black beard?
Did you feel her tummy for hard lumps? Put her on her belly and put your hands underneath her so you're feeling her stomach. Very gently palpate down her right side with your right hand, starting up under her right armpit with your fingers, and work your way down her right side toward her vent, and see if you can feel any really hard lumps or long pieces of poop.