You need to be seen by a doctor. if this pain still persists into tonight and tomorrow, please see your PCP or go to an ER.
Without history it is impossible to say. One common cause of pain in the lower abdomen would be constipation. When you say "lower stomach," the stomach is actually a particular organ.
Are you having any other symptoms? Is the pain relieved with lying down? Have you tried a gas reliever like Gas-Ex or Phazyme (simethicone)? Have you had normal bowel movements? Is there blood in your stool? Is it an odd color? Do you have diarrhea? Are you vomiting? Have an elevated temperature?