Is it possible I can still get pregnant?

Last month I had unprotected sex, but about 8 hours later I took the plan b. Later that evening, I got my period. My period is usually regular. It is always 2 days before my last period. (So last month I got it on the 17th, this month I would get it on the 15th.) My <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a> says I would have got my period on the 13th, but here I am today still no period, should I take a pregnancy test or wait?