Sudden pain in the lower back that makes it hard to straighten up may be caused by muscle spasms, sprains, kidney colic and other reasons. 1. Muscle spasm: it may be caused by dehydration, poor strength and flexibility, muscle fatigue and other reasons. Due to the spontaneous tonic contraction of the muscles, which can cause intense pain, and then there can be a sudden pain in the back to straighten up. 2. Sprain: refers to indirect violence to the limbs and joints around the fascia, muscles, ligaments excessive twisting or pulling, causing injury or tear. Patients may experience severe pain, localized swelling and other phenomena, which may appear in the phenomenon of sudden pain in the lower back to straighten up. 3. Renal colic: It may be caused by renal stone, renal blood vessel infarction, blood clot or necrotic tissue blockage. Due to the strong peristalsis and contraction of the kidneys, severe pain can occur, which can cause the phenomenon of sudden pain in the lower back. Sudden pain in the lower back can also be caused by other reasons, it is recommended that patients go to the hospital in time, follow the doctor’s instructions for treatment, to avoid delaying the condition.