Strains of muscles and ligaments are frequently cause of acute back pain. These can be accompanied by swelling, which is caused by extension and exposure. Strains in sport activities occur easily, when the muscles are used without a warm up or the person is not trained in the activity.
Contusion (bruising) of the tissue and pressure on nerves in swollen areas cause strong radiating pain. If the pain persists for over a week without improvement an x-ray should be taken to determine if a more severe injury like a fracture of the sacrum is present.