According to HealthCentral from Remedy Health Media, there are several possible causes for having two periods in one month. A change in birth control methods can contribute to irregular periods, as can intense exercise, stress, diet and certain medications. However, the most common cause is fluctuating hormone levels.
WebMD reports that irregular periods most often occur during the first few years of menstruation. It can take several years after menstruation begins for menstrual hormones to become balanced and for the periods to become regular. Conversely, older women who are ending their reproductive years may also experience irregular periods, which can include two periods in a month or missed periods.
Having two menstrual cycles in one month or other irregularities is usually not a serious problem. A normal cycle is 28 days. According to Everyday Health, a period is considered to be irregular if it occurs more frequently than 21 days, lasts for more than eight days, does not occur at all for the month or comes early or late. Everyday Health reports that at least 30 percent of women of childbearing age experience one or more of these problems. However, women should see a physician to rule out any medical causes of irregular periods.