There are numerous causes of pain in the lower left side. Pain in the lower left abdomen that radiates toward the back can be caused by pelvic inflammatory disease, Crohn's disease or ulcerative colitis. It can also be caused by pelvic abscesses and kidney stones.The pain could also be simply to do with indigestion or constipation and this can easily be remedied by over the counter medicine.
If the pain persists or you're just concerned, the best thing to do is seek the advice of your local GP. Visiting a doctor might help to catch a serious problem early and be treated with greater efficiency.