What does it mean when your period starts then stops the same day?

Bleeding: It may not be your period. It could be breakthrough bleeding and it could also be implantation bleeding for a new pregnancy. Not all bleeding is your period. It could also be cervical bleeding after sex.

Light period.: The main reason for light periods is a change in hormone levels. Causes for what seems to be a light period can include: thyroid problems, eating disorders, excessive exercise, initial periods for a girl, use of an intrauterine device, oral contraceptives, perimenopause ; implantation bleeding in early pregnancy which rarely occurs.