Anxiety or Panic Attack: This is the most likely diagnosis because taking deep, shallow breaths is a common symptom of anxiety or panic attacks. Patients often exhibit this breathing pattern due to hyperventilation, which can be triggered by stress, fear, or other emotional stimuli.

Hyperventilation Syndrome: Similar to anxiety, hyperventilation syndrome can cause deep, shallow breathing without any underlying respiratory or cardiac issues. It's often related to stress or emotional states.
Early Signs of Diabetic Ketoacidosis (DKA): Deep, shallow breathing (Kussmaul breathing) can be an early sign of DKA, a serious complication of diabetes. Although it's more commonly associated with other symptoms like hyperglycemia and metabolic acidosis, it should be considered, especially in diabetic patients.

Pulmonary Embolism (PE): Although the question specifies no chest pain or respiratory problems, PE can sometimes present atypically, especially in older adults or those with underlying conditions. The sudden onset of deep, shallow breathing could be a subtle sign of a PE, which is a medical emergency.
Cardiac Conditions (e.g., Heart Failure, Myocardial Infarction): While the question rules out chest pain, some cardiac conditions can present with dyspnea (difficulty breathing) or changes in breathing patterns without typical chest pain, especially in women, diabetics, or the elderly.

Neurological Disorders (e.g., Brainstem Lesions, Multiple Sclerosis): Certain neurological conditions can affect the brain's respiratory control centers, leading to abnormal breathing patterns. These are less common but should be considered if other explanations are ruled out.
Thyroid Storm: A life-threatening complication of untreated or undertreated hyperthyroidism, thyroid storm can cause respiratory alkalosis leading to deep, shallow breathing among other symptoms like fever, tachycardia, and agitation.