I have started lolostrin bcp on 12/28 to regulate my period and iron.i had sex the 4th day of my cycle. im 6 days late what is the cause?

Depends: Did you have your period on 12/28? The birth control pills should be started within 6 days of the first day of your period. If you did not start on your period, you may have ovulated and should take a pregnancy test.

Pregnancy test: Do a home pregnancy test. If the result is negative repeat the test 4-5 days later unless you get a period by then. Use first morning urine and follow the instructions for the test carefully.