Plan B and period, it was due today.

My period was due today. So it's a day late. I marked it as late on the calender and it pushed it two days later. I had sex on the 20th and 21st, only using spermicide as protection, he came in me the first time and the second time he pulled out. On the 21st I took plan b in the morning,  under 12 hours. Why is my period late??