It is not. Pain has many sources, including muscle strains. Low back pain is very common and routine imaging is harmful and leads to unecessary care.

Red flags for low back pain include urinary retention, history of cancer, progressive neurologic loss, and the fracture cluster (>50 years old, trauma, history of corticosteroid use).

Many red flags by themselves are poor predictors of serious pathology. https://pubmed.ncbi.nlm.nih.gov/32438853/