I have started lolostrin bcp on 12/28 to regulate my period and iron. I had sex the 4th day of my cycle. I'm 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.