My period is late

My period is late by 3 days, I have already taken a pregnancy test and it was negative. I missed taking my pill for 3 days (went on a trip and forgot it at home) but normally I am really good about taking it on time and me and my boyfriend had sex but he pulled out a little too late. That night I took plan B, that was 4 days before my period was supposed to start. Is it just the plan B that is making it late?