Massive lower gastrointestinal (GI) bleeding (LGIB) is a life-threatening condition requiring prompt diagnosis and management. While diverticular disease is a common cause, rare entities such as colonic submucosal lipomas may complicate the presentation. This case highlights the diagnostic and therapeutic challenges in managing such a scenario, particularly with recurrent bleeding and postoperative complications. A 61-year-old patient presented with recurrent episodes of massive LGIB over a five-day period. Initial evaluations, including CT angiography and nuclear medicine imaging, identified diverticular disease but failed to localize the bleeding source definitively. Despite receiving 15 units of blood transfusions, the patient experienced persistent bleeding, ultimately necessitating emergency surgery. A right hemicolectomy revealed a submucosal lipoma and diverticular disease in the ascending colon with significant intraluminal hemorrhage. Postoperative management was complicated by pulmonary embolism, requiring therapeutic anticoagulation. This case emphasizes the importance of integrating multiple diagnostic modalities when evaluating massive LGIB, especially in patients with uncommon etiologies. It also highlights the complexities of managing massive transfusion protocols and postoperative thromboembolic events in high-risk patients. This report underscores the need for timely surgical intervention in unresolved cases of massive LGIB and the importance of vigilant postoperative care to prevent complications. Enhanced strategies for early mobilization and hydration are critical for improving outcomes in these patients.