I used the "extra blood" line many times, although I downplayed the "not having a baby" because it prompted too much extra conversation among friends who said it ("Are you having a baby this month, Mommy?").
I think the idea of extra blood works fine because it covers you for other situations - for example, when my child accompanied me to the blood donation center, I explained that we have blood to spare and our bodies make more, so we can choose to give some away to people who need it. He loved going and watching the bags collect blood to donate. It demystified blood, took away the fear of it and the fear of needles, and set the stage for various types of giving "of ourselves" and not just money. It helped to emphasize that our bodies have incredible healing power, which covered us for cuts/wounds, illnesses, broken bones, etc.
I don't thin you have to make a big deal of it but just say there are different reasons why extra blood shows up in the toilet, the underwear, the nose, etc. Let her know it's a good idea to let you know if it happens to her anytime but most of the time it's nothing.