When nerves in and around the low part of the spine become pinched, it can provoke pain or cause muscle contractures.
Lumbago is produced when the muscles around the lower back contract, severely limiting movement causing the sensation of being “stuck.” This often occurs from poor postures which overload the spine and cause the muscle surround to contract, protecting the weak zone whether from a sudden improper movement or continued poor posture.
Lumbago can also be caused by:

Scoliosis
Kidney stones
Infections
Endometriosis
Fibromialgia
Pregnancy
Spondylolisthesis
Arthritis
Lumbar stenosis