Lower left abd pain could be caused by multiple issues as Mollyrae stated including,
Appendicitis, Ectopic Pregnancy, Ovarian Cyst, Diverticulitis, PID or Irritable Bowel Syndrome to name a few.
If the pain is actually in your stomach than it could be related to an Ulcer, GERD, Hiatal Hernia or Gastritis, again just to name a few. The abdominal cavity contains many of our vital organs and without a good clinical exam and possibly some additional testing it can be difficult to diagnosis.