Neutropenic Fever: This is the most likely diagnosis given the patient's presentation of fever, myalgia, diarrhea, and chills while on chemotherapy. Neutropenic fever occurs in patients with neutropenia (low neutrophil count), which is a common side effect of chemotherapy. The symptoms presented are classic for an infection in a neutropenic patient.

Infectious Colitis: The presence of diarrhea along with fever and chills suggests an infectious cause, possibly related to chemotherapy-induced mucositis or an opportunistic infection.
Chemotherapy-induced Mucositis: This condition can lead to fever, diarrhea, and abdominal pain due to the inflammation and ulceration of the mucous membranes in the digestive tract.
Sepsis: Although not as specific, the combination of fever, chills, and myalgia in a patient on chemotherapy could indicate sepsis, especially if there's a suspected source of infection.

Typhlitis (Neutropenic Enterocolitis): A potentially life-threatening condition that occurs in neutropenic patients, characterized by inflammation of the cecum. It presents with fever, abdominal pain, and diarrhea.
Clostridioides difficile (C. diff) Infection: This bacterial infection can cause severe diarrhea and is a significant concern in immunocompromised patients, including those on chemotherapy.
Pneumonia: Given the symptoms of fever and myalgia, pneumonia should be considered, especially in a patient with a compromised immune system.

Graft-Versus-Host Disease (GVHD): If the patient has undergone a bone marrow or stem cell transplant as part of their chemotherapy regimen, GVHD could be a rare but possible cause of their symptoms.
Drug-induced Fever: Some chemotherapy agents can cause drug fever, which might be considered if other causes are ruled out.
Hemorrhagic Cystitis: Certain chemotherapy drugs can cause this condition, characterized by bleeding from the bladder mucosa, which might present with systemic symptoms like fever in addition to urinary symptoms.