plan b, pls help.

I took a plan b pill three days ago.. I took the pill one day after unprotected sex. 
I am bleeding like as if I'm having a period. Is this normal? I read something about you could start if you're closer to your fertile window and I just had mine 4 days ago. 
Please help. I'm stressing out. I jjst got off my period one week ago and have only had a week in between this last one and this "new" one. 
Pls help I am seriously stressed.