I've been on Yasmin. we use condoms but it slippe off so I took planB. When wil I start my period?

I've been on my hbc for 4 months now so it has to be effective, but I'm never sure when my periods supposed to start cause it's always different which pill it actuall starts on. Could plan b one step make me late if I took it on the 15th active pill and kept on taking them regularly? I'm scared.