Gastric Lymphoma: Given the patient's history of non-Hodgkin's lymphoma, the development of a new gastric mass, especially one that is ulcerative and necrotic, strongly suggests a gastrointestinal involvement of the lymphoma. The symptoms of anemia, weight loss, vomiting, epigastric pain, and indigestion are consistent with a gastric lymphoma.

Gastric Adenocarcinoma: This is a common cause of gastric masses and could explain the patient's symptoms. The history of chemotherapy could potentially increase the risk of a second primary malignancy.
Gastrointestinal Stromal Tumor (GIST): Although less common, GISTs can present as ulcerative masses in the stomach and could cause the patient's symptoms.

Metastatic Disease: Given the patient's history of non-Hodgkin's lymphoma and recent chemotherapy, there is a possibility of metastatic disease to the stomach from another primary site. This could be from a variety of tumors and would be critical to diagnose due to its implications for treatment and prognosis.
Infectious Causes (e.g., Helicobacter pylori): Although less likely given the description of the mass, infectious causes can lead to gastric ulcers and symptoms similar to those described. Missing an infectious cause could lead to inappropriate treatment and worsening of the condition.

Leiomyosarcoma: A rare type of smooth muscle tumor that can occur in the stomach and present as a mass.
Carcinoid Tumor: A type of neuroendocrine tumor that can occur in the stomach and present with a variety of symptoms, including those described.
Lymphomatoid Gastropathy: A rare condition that can mimic lymphoma but is not a true malignancy, often associated with Helicobacter pylori infection or other chronic inflammatory conditions.