What Causes A Delayed Period While On Birth Control Pills?

Hi, I was supposed to start a new pack of loestrin 24 on the 24th of March, I didn't (I was out at the time) . Instead I started on the 28th of March and took four pills that day... I should've started my period on the 19-20 of April but I didn't. I took a pregnancy test May 3rd- negative... Could the absence of my period be due to the 4 pills I took on March 28th ??