Why Is My Period Only Lasting One Day?

A one-day period can result from hormonal fluctuations, the use of certain contraceptives or excessive stress on the body, says MedGuidance.com. A one-day period with very light bleeding can also indicate pregnancy, as implantation bleeding typically takes place around the same time a missed period is expected to appear. Caused by a fertilized egg attaching to the uterine wall, implantation bleeding typically consists of light spotting, with or without cramping.