Late period questions

So I’m slightly concerned about the fact that my period is late because I’m pretty regular. I had sex a few weeks ago and we used condoms because I was just starting on the pill and I was concerned that my birth control wouldn’t be working right away. The next day I was concerned that the condom broke, so I took Plan B. My period was supposed to come on Saturday, but when it didn’t I thought that the pill was probably just messing a bit with my body, but when it didn’t come yesterday I took a pregnancy test that came back negative. I know that it’s super unlikely that I’m pregnant considering I took all the preventative measures that I could, AND the pregnancy test came back negative, but I’m still a little concerned about why my period is late. Does anyone have any possible answers? I would really appreciate it!