Is there a possibility you may be pregnant? If so, you would have ovulated 7-10 days after your last period and if you were having sex then you could be. If not, don't worry about it right now. Some women are just late for unknown causes. Give yourself another week then call your Dr.
If you have had sex then there is a possibility that you may be pregnant. Some women have irregular periods and if you are late 13 days then I would just wait a bit longer. But if in a couple of weeks you still have not had a period, you should go and see your doctor.