Plan B effect?

I finished my period last Sunday, week ago. I hooked up with this guy Monday but the condom broke and he came in me so we got a plan B pill. We hooked up again Friday and when I woke up Saturday there was blood on my underwear and I thought I was spotting but it’s still here and slightly heavy like a period. I’ve taken a the pill before and this didn’t happen, is it normal? I took the other pill months ago.