Taking plan b period soon

I neeeeeed help. Should I take a plan B even if my period is supposed to come in 2 days? I had unprotected sex but it was with someone I have been for over 4 years. I can't decide whether or not taking it. He didn't have a condom and I have never ever used birth control. Advice? Will it delay me crazily? What should I expect on side effects?