Is it bad to take Plan B one day before my period ?

My boyfriend and I had with a condom but it slipped, and he is afraid that there was some leakage, I am supposed to get my period either today or tomorrow, and am worried that taking Plan B now may throw things off should I not take Plan B ,because of how close it is to my period or should I take Plan B just to be safe, and if so what side effects could Plan B have on my cycle if I take it this close? can someone please help