This can be hormonal.
The tests needed are
sex hormone panel
If your doctor is not concerned, try an endocrinologist.
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This is still most likely premature ovarian failure. It is possible to have another subsequent period when a woman has premature ovarian failure. It is not that anything has changed - it is just that the ovaries will sometimes function again sufficiently to cause a period and even rarely to result in pregnancy.
The ultrasound would not diagnose premature ovarian failure, but would be useful from the perspective of excluding other possible causes of bleeding.
A Gynaecologist can also be helpful in this evaluation if your GP is unable to reach a diagnosis.
Even women with premature ovarian failure and subsequent, there is a point beyond which the woman will not have another period.
We frequently are unable to identify why a women has premature ovarian failure and also why subsequent periods occur.