Nausea/vomiting · tachycardia · unintentional weight loss · Dx?

A 22-year-old woman presented to the emergency department (ED) with a 24-hour history of nausea, vomiting, diarrhea, generalized abdominal pain, and mild headache. She denied shortness of breath, chest pain, or anxiety, and didn't have a history of cardiac problems. The physical examination revealed tachycardia (heart rate, 135 beats/min) and a respiratory rate of 24 breaths per minute.