Answer: Endometrial polyp.
The 3D ultrasound clearly shows a single large feeding vessel passing into a focal endometrial lesion, the classic 'single feeding vessel' sign of an endometrial polyp.
What an endometrial polyp is: a localized overgrowth of basal endometrial stroma with a central blood vessel, covered by surface endometrium, most often arising from the fundus.
Why it bleeds: each polyp has one central feeding vessel with soft stroma and back-to-back glands, and this structure makes it prone to intermittent bleeding, the symptom seen in this patient.
Why not the others: endometrial carcinoma typically shows a thickened, irregular endometrium with multiple feeding vessels and myometrial invasion; a submucosal fibroid is a hypoechoic mass arising from the myometrium with circumferential (not single central) vascularity. Ref: gynaecological ultrasound, endometrial polyp characteristics