If your smears were all fine and all you've had is one episode of light bleeding, with nothing suspicious on your scan just a borderline endometrial lining, and no particular risk factors for endometrial cancer, I would personally be entirely comfortable to just monitor things with a scan at whatever interval your gynaecologist recommends.
Even in the worst case scenario, endometrial cancer is typically quite slow to progress, and you will be kept a close eye on now until this resolves.
Think how many women have a bit of bleeding and don't even notice or go to their doctor.
You have done the right thing but there is an almost irrational fear among some clinicians the way any post menopausal bleeding is immediately presumed to be malignant - the vast majority is not, and most never even comes to medical attention.
Someone above mentioned ovarian - this doesn't generally present with bleeding and they would have got a good look at your ovaries on a pelvic scan,