What Causes Delayed Periods After Taking Postinor?

Period was already delayed (but not pregnant) before I took Postinor 2.I had sex on the 27th of January, he failed to finish outside. I took Postinor 2 45 min later. I was expecting my period on the 11th of January so I m already 17 days late. I m sure I m not pregnant, by coincidence I took a home pregnancy test on the day I had unprotected sex and it was negative.My breasts have been tender for a couple of days, but more so after I took Postinor. Is it possible I m pregnant? Should I take another pregnancy test?