What Causes Delayed Periods?

I started taking trinessa on April 2nd which was four days after my last period had ended. I had unprotected sex on about the 12th of april, but he didn t ejaculate, I know there s precum and stuff, but don t really think I m pregnant. Of course that s always a paranoia, especially since I completely missed one pill losing it, and then on another day forgot and had to make up and take two. I have a period app on my phone that says I should have started my period yesterday. in two days I will get to the sugar pills in my birth control packet and I should get my period then right? I have had mild brown spotting yesterday and today, and was hoping for some peace of mind that that means I am not pregnant and my period is just starting because I started the birth control late. Any help? Thanks:)