I'm on yaz(bcp)for 2.5years&i've never missed a pill.i'm spotting 1week before period.i had unprotected sex with withdrawal method once.can i be preg?

Yes, but not likely: Women who take their birth control pills correctly, are protected every day, all the time. They have no fertile days, so it doesn't matter what added birth control methods are used. Pregnancy is very unlikely. If still worried, one can do a home pregnancy test 3 weeks after sex. Tests are inexpensive and sold in stores everywhere.