As far as I can tell, the halacha discusses three parts of a woman's cycle:
The Niddah flow itself.
The 7 clean days following the flow.
The 11 days following that, during which a woman is assumed Tahor and one who has a flow is considered a Zavah.
What about the rest of the month? Assuming the Niddah flow itself lasts less than a week (which is almost always the case according to WebMD), there are at least 3-5 days unaccounted for.
My current thought is that in those days a woman is not assumed Tahor, and her flow does not make her a Zavah, but I'm not sure.