Spotting

Okay, so this morning I noticed spottings of blood on my underwear in in the middle of my fertile week and I'm not on birth control... So why is there blood? I'm not supposed to get my period for another 2 weeks and I'm pretty regular I'm just a little scared I guess 
I've been googling also and I took a plan B pill last Saturday but it was over a week ago so how could it cause spotting now?