What Causes A Missed Period Despite Taking Lutera?

Hi,
I have been on the birth control pill Lutera for about five months now. For the month of December I had breakthrough bleeding for at least seven days on the second week of the white pills. I know that I get my regular period on the placebo pills. Why didn't I get my regular period during that week? I have been consistent in taking my pills on time and have not missed one. Is this normal?"