How does plan b works?

Princess

Can you get pregnant after using plan b emergency oral contraceptives. I used plan b to prevent pregnancy two days after I had sex. It was my fertile window. The leaflet say u can take it up to 72 hours after unprotected sex. My period is 11 days late not sure if I’m pregnant or if it’s a side effect of the medication. Does any one know how this medication works please ? I’m scared to take a pregnancy test.