New Vet. Ultrasound, full blood screen,and abdominal x-rays ( Both sides and frontal.)
If you rule out physical, medical issues, you may find that she ate a ribbon, or a something flat and thin,and it is very slowly trying work it's way through her.
Blood coming out either end of a dog is a siren going off!
I don't know where you're located, but if you post your approximate geographic location, someone might be able to point you toward a different Vet. If you're in New England, get her to Tufts!!
I don't mean to sound harsh, or critical,but you need a new Vet. Now!
You'll never beat a lesson into a dog, you just beat desire out.