A 52-year-old woman with major depressive disorder has been on sertraline 100 mg daily for 6 weeks with good response. She now presents with complaints of hyponatremia (Na⁺ 128 mEq/L), confusion, and lethargy. Serum osmolality is low (265 mOsm/kg). Urine osmolality is elevated (450 mOsm/kg) with urine sodium 85 mEq/L. TSH and adrenal function are normal. What is the most likely mechanism underlying her clinical presentation?