A 38-year-old woman with known Addison's disease (primary adrenal insufficiency), normally managed with hydrocortisone and fludrocortisone, presents to the Emergency Department with vomiting and diarrhoea for the past 24 hours after suspected gastroenteritis. She feels extremely weak and dizzy. Her blood pressure is 85/50 mmHg and heart rate is 120 bpm. What is the most crucial immediate management step?