has anyone started a new period when you were in your fertile window?

so i have a situation that i’m kinda just looking for a bit of advise or to see if anyone else has had this issue...

i’ve been off my birth control (depo) for over a year now. i’ve recently started having sex again after about two years of celibacy. i was still taking the depo to regulate my period. i’ve had a pretty normalish period since stopping my depo but within the last few months, they have been real irregular. for instance, today was the last day in my fertile window but i just started a new period cycle. this is like the third time this has happened to me, it’s usually not their close to my fertility window. i’ve also taken a couple of plan b pills recently, could that be the cause of the fluctuations?