So, plan B probably would have prevented you from getting pregnant. But, it's a big dose of hormones. These same hormones will also cause you to have irregular bleeding or late periods for 2 to 3 months after taking it. That's all normal. If you want to rule out pregnancy, take a test at 2.5 weeks after the sex episode. But I doubt you are pregnant and this is just a side effect of the Plan B.
This is a confusing post in that you ask about sex the day your period was supposed to start, the least fertile day of the cycle. When else that cycle did you have unprotected? Sex about 10/01/18 with expected period 10/15/18 would have been high risk, and a home test this weekend would probably tell true.