I'm going through exactly the same thing, it seems like the hormones in the plan b pill really messes with one's menstrual cycle. I try to comfort myself by saying that's the reason why my period is late - but I am also going to take a pregnancy test soon to just reassure myself. Plan B is supposed to be very effective, so I don't think you should worry too much (although I know it's very very very stressful, trust me, I even dreamt of getting my period..). I think we should both just calm ourselves, and wait, and take a pregnancy test for extra reassurance.