Jackie is a 57 year old accountant who has a 3 month history of fatigue and has presented to her GP as she is struggling to concentrate at work. She has an appraisal upcoming and admits to increased stress in the workplace. She has little appetite and sometimes skips meals. Despite the stress she says her mood is fine and has an active social life. She denies any abdominal pain, rectal bleeding, or bloating, but admits over the last 6 months she is often constipated. There is no polyuria, polydipsia, or muscle weakness. Past medical history is unremarkable.
Based on the history alone, what is her most likely diagnosis?