plan b pill and late period

Anna • Small town girl, loving life
So I had a mishap last month, it was the first day of my period and my boyfriend and I had Sex with a condom but for the first time ever it broke.. So just incase I took the plan b pill.. Well my period lasted 2 days then stopped for 2 days then started over for 5 days! Finally it ended, well from the last day it stopped, I was due for my period 4 days ago. Now I'm 4 days late... I've take 4 pregnancy test and all were a obvious negative... What is going on? Has this happened to anyone before?? What's causing it? Thanks!