My period is late, but i just started birth control...?

I started taking Yasmin on Dec. 30 on the Sunday Start because my nurse told me to, but my last period was Dec. 7 and it lasted for 4 days. I figured it was way off, but i went with it anyways. My period was supposed to come yesterday, but hasn't... i've been ually active, but can't take a pregnancy test cuz it would be too soon to find anything out... I got on the Yasmin website, and it said for Sunday Start, the pills take effect after a week, and it was exactly a week when i had ... should i be worried? [[sorry for the long story]]