View attachment 11855
Some chumbud already made a period tracker for Yuko, just in case you guys thought I was kidding earlier about her catching some of Gura's more off-kilter viewers.
Also, memberwisp karaoke stream happening now, she seems relaxed and comfortable
Does the tracker say
all the circles are meant to represent the bleeding phase?

Menses = bleeding, secretory = breeding. It makes much more sense for the different-coloured circles to correspond to those separate phases. Which just makes the tracker even more

.
That aside. Headaches, lighter bleeding and whatever else can occur cyclically outside of the menses phase due to the hormonal changes. Some women have that worse than others. Some women also have the complete cycle above over a shorter or longer time frame than 28 days.
Period synchronisation may be real or confirmation bias. If it is real it's not 100%, or you would hear about it a lot more.