Birth control pills / the day after pill

I had sex with my boyfriend without a condom, so I took the day after pill. It's been a week since, and now I'm in my period (not the real one, the one that is caused by the pill). I want to start using birth control pills, is it OK if I start right after this period? Will I already be safe from pregnancy this month? I don't want to wait for my next period to come, because my period is irregular.... My doctor gave me a prescription yesterday and told me to start using the pills today. But I didn't tell her that it's not my real period. What should I do ? When will I be safe from pregnancy this month if I start today? I'm so confused. Please help