Functional abdominal pain: This is a common condition in children and adolescents, characterized by recurrent abdominal pain that is not explained by other medical conditions. The pain is often periumbilical and can be crampy. Given the age of the patient and the description of the pain, this is a likely diagnosis.

Gastroenteritis: Although the patient reports normal bowel movements, gastroenteritis can sometimes present with abdominal pain without significant changes in bowel habits, especially in the early stages.
Urinary tract infection (UTI): UTIs can cause abdominal pain, especially if the infection involves the kidneys. The pain can be periumbilical or flank pain.
Inflammatory bowel disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present with abdominal pain, though they typically also involve changes in bowel habits, which are not reported here.

Appendicitis: Although the pain is initially around the umbilicus, appendicitis typically starts this way before localizing to the right lower quadrant. It's crucial to consider this diagnosis due to the potential for severe complications if appendicitis ruptures.
Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause severe abdominal pain, though it's more common in younger children.
Ovarian torsion: This is a rare but emergency condition that can cause severe abdominal pain. It's more likely if there's a history of ovarian cysts or other pelvic abnormalities.

Henoch-Schönlein purpura (HSP): A systemic vasculitis that can cause abdominal pain, among other symptoms like rash and joint pain.
Meckel's diverticulum: A congenital anomaly of the small intestine that can cause abdominal pain, especially if it becomes inflamed or if there's bleeding.
Abdominal migraine: A condition that can cause recurrent episodes of abdominal pain, often accompanied by other migraine symptoms like headache or nausea.