Musculoskeletal Strain: This is the most common cause of acute low back pain, especially given the history of a recent pull or strain. The pain and spasm are typical presentations of a muscle or ligament injury in the lower back.

Herniated Disc: A herniated disc can cause low back pain, especially if it compresses or irritates nearby nerves. The pain can be severe and may be accompanied by muscle spasm.
Degenerative Disc Disease: This condition can cause chronic low back pain and may be exacerbated by a recent strain, leading to muscle spasm.
Spondylosis or Spondylolisthesis: These conditions involve defects in the vertebrae and can cause low back pain, especially if there is a recent increase in activity or a strain.

Cauda Equina Syndrome: Although rare, this condition is a medical emergency. It involves compression of the nerves in the lower spine and can cause severe low back pain, bowel or bladder dysfunction, and sensory changes in the legs.
Spinal Infection (e.g., Osteomyelitis, Discitis): Infections of the spine can cause severe back pain and are more likely in individuals with certain risk factors (e.g., recent surgery, immunocompromised state).
Spinal Fracture: Especially in individuals with osteoporosis or those who have experienced significant trauma, a spinal fracture could present with acute low back pain and spasm.

Spinal Tumor: Tumors of the spine can cause back pain, but they are much less common than other causes of low back pain.
Ankylosing Spondylitis: A type of arthritis that can cause chronic back pain and stiffness, particularly in younger individuals.
Referred Pain from Visceral Sources: In some cases, pain from abdominal organs (e.g., kidney stones, pancreatitis) can be referred to the lower back, presenting as low back pain.