Could be a variety of things. Your stomach is actually on your left side. Your pancreas is on that side. The sigmoid colon is low, but on that side, so if you're having problems related to the constriction of the sigmoid, "material" backup can cause pain in that region. My diverticulitis causes pain a bit lower, but on the left side as well. If there is any fever associated with the pain, get thee to a doctor quickly.
Chris