Any and all kinds of vaginal or uterine bleeding are not a menstrual period, which is specifically the sloughing off of the uterine lining, which is needed to sustain a pregnancy, and generally only occurs when, and because, someone is not already pregnant.
A lot of the time, the inconsistency here is that some people use the term period to describe all kinds of vaginal or uterine bleeding, which is incorrect.
Just FYI, the American Pregnancy Organization you linked to is a hidden antichoice organization. More here, if you like:
http://www.scarleteen.com/blog/heather_ ... on_on_cpcs Because of that, not only do we strongly discourage anyone from using them because we support people's rights, but their information is always going to lack credibility, because they already do publish false or misleading information about abortion, a medical service.