Dr. Heidi A. Fowler on HealthTap explains that a short, light period can mean many things, from a change in hormone levels to the start of perimenopause. Women with thyroid problems and eating disorders also can experience irregular periods.
WebMD points out that there is no "normal" period. Some women naturally have shorter, lighter periods, while others have heavier ones, and periods in the same woman tend to vary from month to month. The most important considerations are significant changes to a period and the age of the woman. For example, an older woman who notices a change in her period, such as it is becoming light and short, may be undergoing perimenopause. Perimenopause occurs before menopause and is a response to dropping estrogen levels.
Another reason for short, light periods is excessive exercise, WebMD states. Exercise influences the rate and amount of hormones that are produced. This phenomena, exercise-induced amenorrhea, is common in ballerinas and runners.
According to WebMD, light, short periods can also signal pregnancy. Birth control is not guaranteed protection. Dr. Fowler mentions that sometimes light bleeding occurs during early pregnancy when the fetus implants in the uterine wall. Also, short menstrual cycles can occur because of the use of contraceptive drugs or devices.