A 41 year old patient presents with very light intermenstrua…

A 41 year old patient presents with very light intermenstrual bleeding, but otherwise normal menstruation. She is not taking any medication. All labs are normal and a pelvic ultrasound ruled out fibroids and also showed a very thin endometrial stripe of only 1 mm. Her past medical history is unremarkable, with no chronic disease, hospitalizations or surgeries. Her vitals are normal and she weighs 115 lbs and is 5’6″. The most likely diagnosis is: