Pyelonephritis: This condition, an infection of the kidney, can cause lower back stiffness, stomach discomfort due to referred pain, and an elevated temperature due to the infection. The combination of these symptoms, especially if accompanied by urinary symptoms like dysuria or frequency, makes pyelonephritis a strong candidate.

Appendicitis: Although classically presenting with right lower quadrant abdominal pain, some cases of appendicitis can cause more generalized stomach discomfort and lower back stiffness, especially if the appendix is retrocecal. An elevated temperature would be consistent with an infectious or inflammatory process.
Diverticulitis: Inflammation of a diverticulum in the colon can cause abdominal pain (often left lower quadrant), fever, and potentially lower back stiffness if the inflammation involves adjacent structures.
Gastroenteritis: This condition, often caused by a viral or bacterial infection, can lead to stomach discomfort, fever, and potentially lower back stiffness due to systemic inflammation or dehydration.

Kidney Stones with Infection (Septic Urolithiasis): While kidney stones can cause severe pain and lower back stiffness, if an infection is also present, it becomes a medical emergency due to the risk of sepsis. An elevated temperature in the context of known kidney stones should prompt urgent evaluation.
Pancreatitis: Especially if the stomach discomfort is severe and radiates to the back, pancreatitis should be considered. An elevated temperature could indicate an infectious component or necrotizing pancreatitis, both of which are serious conditions.
Ectopic Pregnancy: In women of childbearing age, ectopic pregnancy can present with abdominal pain, fever (if there's associated infection), and lower back stiffness. This is a life-threatening condition that requires immediate diagnosis and treatment.

Inflammatory Back Pain Conditions (e.g., Ankylosing Spondylitis): While these conditions can cause lower back stiffness and potentially systemic symptoms like fever, they are less likely to cause acute stomach discomfort and elevated temperature simultaneously.
Endocarditis: A bacterial infection of the heart valves can cause systemic symptoms including fever and potentially abdominal pain due to embolic phenomena. However, it's less directly associated with lower back stiffness and stomach discomfort as primary complaints.
Systemic Lupus Erythematosus (SLE) Flare: SLE can cause a wide range of symptoms including abdominal pain, fever, and musculoskeletal complaints like lower back stiffness. However, diagnosing SLE or an SLE flare based on these symptoms alone would be challenging without other supporting evidence of autoimmune disease.