Starting period immediately after beginning BC?

[http://users.livejournal.com/_crystalmyth/]

I'm asking this on behalf of a friend. She started HBC for the first time this past Sunday, which (der) makes her a Sunday start. However, this wasn't the first Sunday after her period. In fact, it was the Sunday that she was supposed to get her period, apparently, because she got it that night. Normal, right? The HBC wasn't in her system long enough to ward off her period, right? Would this possibly make her a first-day start?