The average menstrual period length is 2-8 days. But some contraceptives and underlying health issues may be responsible for bleeding beyond the average days. My questions are: did you change your birth control method of recent to a new one, are you menopausal, or is there any gynaecological condition you are aware of? If no, I will suggest you see a doctor for proper investigation.
Also, as this is happening for the first time, try and keep record of the next month period incase it changes back to normal or remains the same. Whichever one it is, seeing a doctor should not be undermined.
I hope this helps.