A 52-year-old woman with a 15-year history of type 2 diabetes mellitus presents with recurrent episodes of hypoglycemia despite a stable dose of metformin and glibenclamide. Her fasting blood glucose is 85 mg/dL, but she experiences sweating, tremor, and confusion 2–3 hours after meals. A mixed meal tolerance test shows: fasting glucose 90 mg/dL, 2-hour glucose 140 mg/dL, fasting insulin 18 mIU/L, and 2-hour insulin 65 mIU/L. Fasting glucagon is 120 pg/mL (normal 50–100 pg/mL). Which of the following best explains her hypoglycemic episodes?