I've heard that exhaling as your right foot hits the ground will help a prevent side stitch on the left side or will help one in progress go away. If you were running downhill, that can cause stitches (or cause the pain to become more intense).
But I'm not sure what's going on with you. Stitches on the left side are pretty common but if it was a normal stitch I wouldn't expect you to still feel pain today.