The origin of pain can either be in the abdominal muscles or inside the abdomen. The place which you describe can become painful in case of an abscess or an infected hematoma in abdominal muscles. It could also be a periumbilical hernia that has got trapped resulting in small bowel obstruction or mesenteric ischemia. Early appendicitis too behaves this way and so does colitis. It could also be cystitis, urinary tract infection, diverticulitis or inflammatory bowel disease.
It is difficult to comment beyond this at this stage. A CT scan can tell for sure what the swelling is. Please consult your PCP for primary examination followed by proper referral.