Periumbilical pain can have several causes:
Mesenteric adenitis
Abdominal aortic aneurysm
Appendicitis in the initial stage
Diabetic ketoacidosis
Aortic dissection
Umbilical/periambilical hernia
Gastroenteritis
Mesenteric ischemia
Incisional hernia
Inflammatory bowel disease
Intestinal obstruction
Pancreatitis
Mesenteric thrombosis
Uremia