Ovarian Cancer: Given the significant weight loss, anemia, abdominal and back pain, and abnormal fullness in the right lower quadrant (RLQ), ovarian cancer is a strong consideration. The symptoms and signs, especially the rapid weight loss and abdominal fullness, are classic presentations for advanced ovarian cancer.

Colorectal Cancer: Weight loss and anemia are common in colorectal cancer, and abdominal pain can occur, especially if the tumor is in the right colon, which could explain the RLQ fullness.
Gastrointestinal Lymphoma: Can cause weight loss, anemia, and abdominal pain. The fullness in the RLQ could be due to a mass or enlarged lymph nodes.
Pancreatic Cancer: Although it typically presents with pain in the upper abdomen radiating to the back, significant weight loss, and possibly anemia, it's a consideration given the broad range of abdominal symptoms.

Appendicitis or Other Acute Abdominal Conditions: Although less likely given the chronic nature of symptoms, conditions like appendicitis can present atypically in older adults and can be life-threatening if not promptly treated.
Intra-abdominal Infection or Abscess: Could explain the abdominal pain and fullness, especially if there's a history of recent surgery, infection, or immunocompromised state.
Bowel Obstruction: Can cause severe abdominal pain, vomiting, and constipation. If not recognized and treated, it can lead to serious complications.

Carcinoid Tumor: Can cause weight loss, abdominal pain, and anemia, but typically presents with flushing and diarrhea due to serotonin secretion.
Abdominal Tuberculosis: Can cause weight loss, abdominal pain, and anemia, but is less common in this demographic without specific risk factors.
Primary Peritoneal Cancer: Similar to ovarian cancer but arises from the peritoneum itself. It's rare but should be considered in the differential for unexplained abdominal symptoms and weight loss in postmenopausal women.