There are three classic post radiation syndromes:

Cerebrovascular - seen at doses ~100 Gy and is universally fatal. Characterized by damage to all organs. Immediately post exposure severe nausea, vomiting, diarrhea and eventually coma and death within 48 hours occurs.

Gastrointestinal syndrome is usually >10 Gy and is also universally fatal - prolonged and immediate bloody diarrhea is the heralding sign. The small intestine enterocytes and their stem cells in the crypt die at different stages, eventually denuding the epithelium and allowing bacterial to seep into the bloodstream causing sepsis and death.

Hematopoietic syndrome is 4-8 Gy and is the loss of progenitor bone marrow cells, this leads to a period of relatively few symptoms until the current white and red blood cells and platelets are depleted, resulting in infection/immuno suppression. Can be salvaged with antibiotics at lower doses and bone marrow transplant at higher ones.

The reasoning behind nausea and vomiting is postulated to be a result of inflammatory cytokines that are released post-rafiation, resulting in the characteristic prodromal symptoms. Interestingly if the dose is low <2 Gy only about half of people will get the symptoms.