Question about pregnancy and using the 21 day pack?

I use the 21 day Yasmin contraceptive pack. My cycle starts on sunday, so after 21 day i'm supposed to get my period on sunday. However, I usually get my period around tuesday or wednesday (though it ends on time, around next sunday). I was wondering if there is the possibility of me getting pregnant in the time span between the day i'm supposed to get my period (sunday) and the day that I actually get it.