Sharp pain in left si joint when bending, turning and moving leg out, plus muscle spasms at night, no disk prombles, could it be si joint unstable?

SI instabil Unlikely: Sacroiliac joint instability is unlikely. Have you had severe low back trauma or autoimmune disease involving your spine? More likely, you suffer from acute strain as in strenuous exercise, poor posture or poor body mechanics in your daily activities. Evaluation by your primary medical provider or specialist, or physical therapist might help.