Plan B is 89-95% effective after a single episode of unprotected sex (Source :
http://healthcenter.uoregon.edu/pati...mergency.html).
It does contain a high dose of hormones which
can throw your cycle off a little.
HPT's should typically be taken 2-3 weeks after a missed period, using your first morning urine. So if you do not get your period within the next 2-3 weeks, I would say take an HPT.
As Glinda said, there are a number of things that can cause a late period, such as stress and diet.