You can take a pregnancy test to be sure. There are two possibilities that can help explain why you got your period so quickly. The first possibility is that you have irregular menstrual cycles (each cycle lasts a different amount of time). Irregular menstrual cycles are usually caused by some hormonal disturbance in the axis between the hypothalamus, pituitary gland, and/or the ovaries. The second possibility is that either this period or your last period is not a real period (menstruation) but another kind of vaginal bleeding (spotting). Reasons for spotting are various and can be either organic or functional. You can follow up by scheduling an examination with your gynecologist.