This is the 8th Day I've taken the yasmin birth control pill?

My doctor told me when she prescribed me this birth control that it is NOT true that it takes a month or two weeks or anything for a birth control pill to become effective. She says with more research they have found now that it starts to work within the first couple of days...I had with my boyfriend today, the 8th day of the pill, which i have been taking on time regularily everyday, and it was unprotected. We just had a baby two months ago, is it possible even though I was told that the pill works right away by my doctor that I could become pregnant? I am also still nursing my 2 month old. Anyone have any comments?