No, many can feel it beforehand, and you notice it when you go to the bathroom before as well, as certain things change their properties slightly, it's not a "nothing" phase and then "floods out of your body".
It's borderline parody how little education there is for males when it comes to things like this.
You seem to forget that there is about a third of the day when shockingly enough a person is not conscious to feel blood leaking out of them. I mentioned staining furniture for a reason - I promise you that menstruation does not wait for you to wake up to start.