chance of pregnancy?

I had sex last saturday, no form of birth control was used.. 3 days after i went to the doctor && got put on the pill. they had me take a urine test to make sure i wasn't pregnant but i think it might have been to early to tell. ive been taking the pill everyday since. My period is off, could it be from the birth control? i am taking a pregnancy test this saturday. is there a big chance that i am pregnant?