What Causes Abdominal Pain Below The Naval?

My 18 year old daughter, who has a high tolerance for pain and does not complain often, is complaining of abdominal pain below the naval. Says it has been going on for at least a year. Says she is always aware of some type of pain there, sometimes mild, but sometimes sharp. Says it is definitely worse when she wears tight low-rise pants. As for food she eats a lot of salad, bananas and peanut butter, raisin bran cereal, granola bars. Doesn't associate the pain with stress. Was diagnosed with an umbilical hernia when she was 2 or 3, seen by a surgeon but told surgery not needed at the time and no issues with it since then. A year ago she saw a nurse practitioner who had a blood test done and recommended Zantac (which she did not take because she didn't feel the pain was in her stomach or esophagas.)