Irritable Bowel Syndrome (IBS): Given the chronic nature of abdominal pain, nausea, and the absence of weight loss, IBS is a plausible diagnosis. It's a common condition in young females, often associated with stress and sleep disturbances.

Gastroesophageal Reflux Disease (GERD): Symptoms like abdominal pain and nausea could be indicative of GERD, especially if the pain is localized to the upper abdomen and worsens with food intake or at night.
Functional Dyspepsia: This condition involves recurring signs and symptoms of indigestion that have no obvious cause. Symptoms can include pain or discomfort in the upper abdomen, early satiety, and nausea.
Stress and Anxiety: Given the patient's difficulty sleeping and the chronic nature of her symptoms, stress and anxiety could be contributing factors or even primary causes, possibly leading to or exacerbating gastrointestinal symptoms.

Appendicitis: Although less likely given the chronic nature of symptoms, appendicitis can present atypically, especially in females. It's crucial to consider this diagnosis due to its potential for severe consequences if missed.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with abdominal pain, nausea, and changes in bowel habits. While weight loss is a common symptom, it's not universal, especially in early stages.
Ovarian Cysts or Torsion: These conditions can cause abdominal pain and nausea. Ovarian torsion is a surgical emergency, making it a "do not miss" diagnosis.
Ectopic Pregnancy: Although the patient's weight has been steady, any female of childbearing age with abdominal pain and nausea should be screened for pregnancy, including the possibility of an ectopic pregnancy, which is a medical emergency.

Celiac Disease: An autoimmune reaction to gluten, leading to small intestine damage, which can cause abdominal pain, nausea, and malabsorption. It's less common but should be considered, especially if other symptoms like diarrhea or fatigue are present.
Gastric or Intestinal Lymphoma: These are rare cancers that can present with nonspecific symptoms like abdominal pain and nausea. They are less likely but important to consider in the differential diagnosis due to their severity.
Endometriosis: While more commonly associated with pelvic pain, endometriosis can cause a wide range of symptoms, including abdominal pain and nausea, especially if the endometrial implants are located near the intestines.