Sometimes stress, or hormones (maybe even those from the pill?) can cause your period to be off. How long have you been on the pill? Do you take it at the same time EVERY day? Have you missed any pills in the past month or so? Have you had unprotected sex (without a condom or other bc method)? Its also possible that you can get pregnant on the pill, but the only way to know would be to take a HPT or to see your Dr for a blood test. I would recommend taking a First Response Early Result test if you decide on the HPT. If it is negative and you still don't start within another week or so I would put a call into your Dr. Good Luck!
A lot of things can delay a period. Since the pill has a small failure rate it would be a good idea to take a pregnancy test to rule that in/out.