Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Maddy has hemolytic anemia. I understand that she is horribly weak, not eating and is very lethargic.
It makes sense that if she has very few red blood cells left her ability to carry oxygen to her brain, muscles, and digestive tract will be very poor. That being the case she will be too weak/tired to move and won't feel like eating. And whatever caused her to develop hemolytic anemia may be playing a part in her symptoms.
It is important to support her while tests are done to try to figure out why she is so sick. That means medications to suppress the attacks on her red blood cells, possibly a blood transfusion, and intravenous fluids and antibiotics.
Red blood cells can be destroyed due to tortuous blood vessels in a tumor, or tick borne diseases, or autoimmune (body attacking itself) disease, which is sometimes triggered by cancer.
I do recommend blood tests to look for tick borne diseases, a coomb's test to look for autoimmune disease, as well as a blood smear to look for young red cells to see if her anemia is regenerative.
Because she is an older girl an abdominal ultrasound to look for cancer is a good idea.
Prognosis and treatment will depend upon what is causing her red cell damage.
In the meantime she won't feel better overnight even with appropriate treatment. It will take at least 3-4 days for her body to start producing new red blood cells in enough quantity to feel better and for the medications to really start working. That's why sometimes we need to give them one or more blood transfusions to keep them going while we wait for them to respond.
I know it is hard to see her the way she is now, but depending upon what the tests find she may just need more time.
Please let me know if you have any further questions.