plan b short period?

I took plan b after having unprotected sex on an ovulation day. He didn't come in me he was just worried about pre come. I took it about two weeks ago and I just ended my first period since taking it and it was only 4 days..is that normal? Would mono affect it? I just got diagnosed with mono after having symptoms for the past week and a half. Help? My periods usually last 7-9 days