I took plan b once and it worked for me but it gave me two periods in one month. & your period could be delayed so just try not to worry too much if that happens . You're probably not pregnant.
Bc and plan b aren't 100% able to prevent pregnancy but they work for the most part your probably not pregnant but you'll have to wait til your next period to know.
There's always a possibility especially if it was unprotected sex . If you took the plan b right after the sexual contact you should be okay its good I think up to 72 hrs . If your period is late all that can be said is to take a test meanwhile try not to stress that may delay your period goodluck . :)