Irritable Bowel Syndrome (IBS) with referral pain: The combination of constipation, alternating diarrhea, and abdominal pain, particularly the pain pattern described, could suggest IBS. The referral pain to the left hip and the specific posture (sitting with the left leg externally rotated and flexed) that exacerbates the pain might be related to the gastrointestinal issues due to the complex innervation of the abdominal and pelvic regions.

Gallbladder Dysfunction: Despite a normal gallbladder ejection fraction, the epigastric pain described could still suggest gallbladder dysfunction, especially if the pain is exacerbated by fatty foods or occurs after eating. However, the normal ejection fraction reduces the likelihood.
Musculoskeletal Issues (e.g., Piriformis Syndrome or Trochanteric Bursitis): The left anterior hip pain, especially when sitting with the leg in a specific position, could indicate a musculoskeletal issue. Piriformis syndrome or trochanteric bursitis could cause pain in the described area and might be exacerbated by certain positions.
Gastroesophageal Reflux Disease (GERD): Although the patient denies esophageal symptoms typically associated with GERD, atypical presentations can occur, and the epigastric pain could be related to GERD, especially if it's exacerbated by food or position.

Cardiac Ischemia: Although the pain pattern is not typical for cardiac ischemia, and the gallbladder ejection fraction is normal, cardiac issues can sometimes present atypically, especially in diabetic or hypertensive patients. The morning headache could be unrelated but warrants consideration of cardiac causes due to the potential severity.
Abdominal Aortic Aneurysm: This is a less likely cause of the patient's symptoms but is crucial not to miss due to its high mortality rate if ruptured. The epigastric pain could potentially be related to an aneurysm, although other symptoms like a palpable mass or back pain are more typical.
Pancreatitis: Acute or chronic pancreatitis could cause epigastric pain and might be considered, especially if the pain is constant and severe, although the normal gallbladder ejection fraction and lack of other typical symptoms (like radiating back pain) make this less likely.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could cause a mix of gastrointestinal and extraintestinal symptoms, including joint pain and headaches, but are less common and typically present with more pronounced gastrointestinal symptoms like bloody diarrhea.
Amyloidosis: A rare condition that can cause a wide range of symptoms, including gastrointestinal issues, joint pain, and headaches, but is much less likely and would typically be considered after other diagnoses have been ruled out.
Neurological Disorders (e.g., Multiple Sclerosis): Could potentially explain some of the varied symptoms, especially the morning headaches and possibly the hip pain if related to neurological rather than musculoskeletal issues, but would be an uncommon presentation.