Gastroesophageal Reflux Disease (GERD): This condition can cause palpitations or a sensation of fluttering in the chest, which may be perceived at the level of the diaphragm due to the proximity of the esophagus to the diaphragm and the potential for referred pain.

Anxiety or Panic Disorder: These conditions can cause palpitations that may be felt in various parts of the chest, including the area near the diaphragm, due to increased heart rate and awareness of heartbeats.
Atrial Fibrillation: An irregular heartbeat that can cause palpitations, which might be perceived in the chest area, including near the diaphragm, especially if the patient is highly attuned to their body sensations.
Mitral Valve Prolapse: A condition where the two valve flaps of the mitral valve do not close smoothly or evenly but instead bulge (prolapse) upward into the left atrium when the heart contracts. It can cause palpitations and is often associated with chest discomfort.

Myocardial Infarction (Heart Attack): Although less common, a heart attack can present with atypical symptoms, including palpitations. It's crucial to consider this diagnosis, especially in patients with risk factors.
Pulmonary Embolism: A blockage of an artery in the lungs that can cause sudden and severe symptoms, including palpitations, and is a medical emergency.
Cardiac Tamponade: Fluid accumulation in the sac around the heart (pericardium) that can compress the heart and impede its function, leading to palpitations among other severe symptoms.

Pericarditis: Inflammation of the pericardium, the sac surrounding the heart, which can cause chest pain and palpitations.
Hypothyroidism: Although more commonly associated with fatigue and weight gain, severe hypothyroidism can lead to cardiac issues, including palpitations.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of palpitations, hypertension, and other symptoms due to excessive catecholamine release.