What Causes Missed Periods?

I have been on microgestin 1/20 since April 2013. Throughout the year it seems like every other month I start bleeding a week after taking my active pills, sometimes I get my period during my week off, sometimes I don t. Beginning of May I started bleeding early (my gyno says it s normal breakthrough bleeding) and I didn t have my normal period when it was due, no problem. Now this month I didn t get my period either. Should I be concerned? Should I take a pregnancy test even though I take my pill within 10 minutes of my set alarm every day religiously?