Infection in bowel is the reason for the intensive bleeding, but Doctors are not sure the cause for that. There is a good possibility that this is GVHD (Graff vs. host desease) which is a common situation in which the new bone marrow attacks the host body. In low level it is encouraged as it attacks the remains of Leukemia. But in sever mode it is very dangerous and can attack the skin, liver or bowel, which seems the case with Sivan.
In Hadassah Jerusalem hospital they have experimental treatment for that and in case needed, they might use it.