plan b spotting? help please

So I took a plan b two days ago but I've been having unprotected sex all week but I finally got my hands on a plan b and I spotted today very light to the point where it wasn't that noticeable I have take plan b a year ago just once this is my second time ever but this hasn't happened before and my period is until a week and a half from now I know my period cab come a week early but it's almost two weeks away and I was fertile during that time but I know plan b is pretty effective i just need to know if I'm over stressing this