If the pain is in the left lumbar region (see the quadrants of abdomen—a google image should get you this), then this could be due to stomach issues (like acidity), it may be due to
pancreatitis, or spleen issues or kidney stones or kidney
infection or due to urinary infection or colitis. An ultrasound of the abdomen followed by urine and stool examination may help clinch the diagnosis. You may need additional tests like CT scan of abdomen, serum amylase, lipase etc.