There is no reason to be concerned about an early period. Unless you could be pregnant, this is to be expected every so often as you get older. It is due to a decrease in progesterone that makes the lining of the uterus less stable. It can cause an early period or abnormal bleeding It is not due to stress and doesnt need to be specifially fixed
A 20 day cycle is not abnormal as long as it is sporadic. Cancer would be very rare in your age. These are perimenopausal symptoms. There is no reason to be worried sick over a single early period.