Some women miss periods while on hormonal birth control, even when they are not pregnant. This is not abnormal or dangerous (although it is a little anxiety producing!). However, if you go 2 or more months in a row without a period, or you miss your period after a month where you did not use all of your pills correctly, or you have symptoms associated with pregnancy, such as morning sickness or unusual breast tenderness, you should take a pregnancy test.
The placebo week is when you “should” normally get your period, though your period may not line up perfectly with the placebo pills. For instance, your period may start on the 3rd or 4th placebo pill day and may last through the first couple days of the new pill pack. You should start your new pill pack the day after you take your last placebo pill, even if your period is still going.