Hello Doc, I Had My Period Between 7th And 10th

Hello Doc,
I had my period between 7th and 10th of last month August, I also had sex on the 31st (which is supposed to be a safe period) of last month after which I used postinor 2 immediately after the intercourse but my period is some days late. It was supposed to come on the 4th Sept but It has not. I had a pregnancy test just in case this morning 9th of September and its negative.
Please what could possibly be the issue?

Thank you.