Why i dont still have my period? Do i need to consult a doctor? Or use pregnancy test again?

Hi, I just want to ask if it is possible to get pregnant if my due date is 6th of the month, then on that night my husband and I had our intercourse, I don't use contraceptive pills, I only use the calendar method, what my husband did is the pull out method, than after he pulled it out, he used condom again to release the sperm..Until now, 9 days ago, I don't still have my menstruation..I took pregnancy test after 5 days when we had our intercourse, which means, 6th is my due date of menstruation and I took PT on the 11th day which turned out NEGATIVE..please help, I just want to know what happened and until now I don't have my monthly period..this is the first time that my menstruation delayed, usually it falls on my due date, but now Im wondering why i dont still have my period. do i need to consult a doctor?or use pregnancy test again?

thanks in advance for those who will reply.