I took my third placebo pill yesterday and still no period?!!?

Okay I have been on birth control since my freshman year for acne and irregular periods. I went off it my junior year and just recently started taking it again (senior now) because I started getting bad acne again. Im on my second pack and me and my boyfriend had unprotected a week ago (Wednesday) and I took my first placebo pill Thursday night and I havent taken my fourth one which is due around 10 tonight. Im scared that Im pregnant.. Has this happened to anyone else? Im on Ocella birth control (generic for yasmin) I ALWAYS would start on the third inactive pill??