Do you know where the blood came from? Was it from your bowels,bladder as or your vagina? If you are sure that it was vaginal bleeding then that sounds like your period. Congratulations and welcome to womanhood! Periods can start really lightly or briefly and are often irregular for the first year or so. Don't worry and mark your calendar to start tracking your cycles. If you aren’t sure where the blood came from, it might be best to see your GP.