I really hope you can identify the source of that blood in the stool through the ultrasound.
FWIW, mine had red blood in his stool (often bloody diarrhea) when his food allergies were at their worst. The inflammation created polyps that bled. His anal glands were also constantly impacted and infected from the inflammation of the area. His rectum was always bright red, and he was constantly licking it. It was an ordeal getting all that sorted out, but once we found food that he could digest without it lighting up his GI tract, the inflammation went away, and with it all of the other problems.
We had polyp removal surgery, anal gland antibiotics, and appointment after appointment. Finally my vet basically started thinking out loud about a hunch that a severe food allergy could be behind all of it -- and it was!
While your vet is doing the ultrasound, maybe ask if he or she can also get a really good look at the anal area, and check it for inflammation. It's one more puzzle piece in trying to figure out what's going on with this poor dog.
There are so many things that could be going on -- I really hope it's one that you're able to manage and treat to make her feel more comfortable!