Maybe it's your appendix. Definitely go to the doctor or hospital. Could you have hurt yourself working out? Like a pulled muscle or just sore. If you are a woman maybe your extra sensitive and are feeling yourself ovulate. Also, possible bladder infection. Definitely don't brush it off and see someone if it becomes severe and constant.
Could be the result of a bruise to that area. Internal organs located around that area are the gall bladder, appendix, and kidneys towards the rear. Also, fractured ribs may be a cause.
Lower, left abdominal pain can be caused by an ovarian cyst, bowel obstruction, diverticulitis, constipation, hernia, endometriosis, kidney stone, irritable bowel syndrome or gas.