That happened to me after stopping the pill to ttc our second baby- my period was two weeks later than it should have been- I totally thought I was pregnant for a week, but tests came back neg. After that, I figured i'd start trying again, and got my period !
My cycle must've still been a abit whacked the next month, because that period only lasted three days, and I got pregnant, like two days after it finished when I shouldn't have been fertile for at least another 9 days.
Your body just doesn't know what to do with itself when all those hormones it's gotten used to getting suddenly stop coming.