plan b doesn’t fail right?

About exactly 4weeks ago i had unprotected sex with my boyfriend. at the time we didn’t have condoms that he fit in so we improvised and went without.

within 3 hours of that sex i took a plan b with a red bull (not my smartest drink choice but hey the pill was ingested so i’m good?)

since then i’ve been pretty okay except i have had a few mood swings that are different from my usual mood. i also just had the shortest period of my life and i’m not sure why... i’m always super heavy for at least 4 days and this time i was heavy day one and then spotting day two, then blam period finished.

i’m pretty sure i’m not pregnant since im not exactly a healthy person, meaning my periods are hardly regular. i just want to clarify that plan b is pretty hard to get pregnant with?

thanks for listening!