Can I be pregnant after having an abortion even though I'm on the birth control pill?

I had an abortion 3 weeks ago. It was on a Monday, so that following Sunday I started taking my birth control pill. I've now been taking it for a little over 2 weeks. I was told to wait 3 weeks before having sex again, which I did. Now I'm being told the birth control pill doesn't actually start working for a month, rather than right away. Does this mean that since it's very easy to get pregnant after an abortion, that I probably am?