Negative pregnancy test but really light period?

Not a period?: It might be that this bleeding is not, in fact, related to your period, but rather breakthrough bleeding occurring during a cycle where you did not ovulate this is common and women who have anovulatory cycles often present with absent cycles or unusual, confusing breakthrough bleeding. Talk to your doctor about working through this problem.

Light period: The main reason for light periods is 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.