plan b problems?

so i took the generic plan b for the first time recently, and i have been feeling kinda weird since. i am supposed to start my period in like 4 days but i’ve been getting cramps and feeling like it’s gonna start sooner. is that normal? how do i know if it worked? i know the package said that it could mess with your cycle, but i’m not sure exactly what that means 😅 am i supposed to get my period at the same time or will it come sooner? i did read all the info i just don’t know how to interpret it