Hello, There are multiple reasons for the pain on your left side of your back.Some of the common causes are Radiating pain from- portruded disc bone disease of the lumbar vertebrae Kidney stones/cancer rib cage injuries splenomegaly pancreatitis inflammation of intestines stomach ulcers To get a proper diagnosis you need to tell your doctor any other prevailing symptoms like Pain on urination hyperacidity before or after eating changes in stool colour breathing difficulty digestion difficulty vomiting, headache,fever,cough,cold,pain on any other area swelling in legs or any other area depending upon the diagnosis the doctor will suggest appropriate blood test and other investigations like ultrasound,MRI or X-ray Hope you found your answer thank you