Musculoskeletal strain: This is the most common cause of back pain, often resulting from overuse, poor posture, or injury. The pain is typically localized to the area of strain and may radiate to the left side.

Kidney stones: Pain from kidney stones can manifest as severe, sharp pain in the left side of the back, radiating to the groin area. It's often accompanied by nausea, vomiting, and frequent urination.
Herniated disk: A herniated disk in the lumbar spine can cause pain on one side of the back, depending on the location of the herniation. This pain can be severe and may radiate down the leg.
Degenerative disc disease: Wear and tear on the spinal discs can lead to chronic back pain, which may be more pronounced on one side.
Scoliosis: Abnormal curvature of the spine can lead to uneven muscle strain and back pain, potentially more severe on one side.

Pulmonary embolism: Although less common, a pulmonary embolism can cause sudden, severe back pain, especially if the embolism is located in the upper lobes of the lungs. This is a medical emergency.
Aortic dissection: A tear in the aorta can cause severe, tearing pain in the back, which may radiate to the left side. This condition is life-threatening and requires immediate medical attention.
Pancreatitis: Inflammation of the pancreas can cause severe, persistent pain in the upper abdomen and back, which may be more pronounced on the left side.

Spinal tumors: Tumors on the spine, whether benign or malignant, can cause back pain that may be more pronounced on one side, depending on the tumor's location.
Osteomyelitis: Infection of the bone can cause pain, tenderness, and swelling in the affected area, which may be localized to one side of the back.
Epidural abscess: A collection of pus in the epidural space surrounding the spinal cord can cause severe back pain, fever, and neurological deficits, which is a medical emergency.