Well Spoilers..
She was the 16th princess and she no maids because no one took care of her. By the time she had menstruation . She didnt know what to do and she was afraid . Thenshe took this medicine that could stop her from menstruating. But its a healthy way. Her fallopian is so clean because of it. And so many more twist which i cannot explain.
But if i were you. I would read the novel. Cause damn. Theres alot of missing action in manga. The novel is wayyy to descriptive. And this has so many chapters and slow. I urge you to read the novel.
She doesn't have an irregular period. She just simply ate mugwort, an herb that stops bleeding, on her first period. And then when she saw how it stopped the bleeding she ate it whenever her period came for about a year I believe. She remembered that women before ate mugwort inorder for them to not have children with savages. So eating mugwort makes a woman barren or infertile. Yes it does make a woman infertile, but with the right antidote or medicine she will be able to carry children. In the story when a woman eats mugwort it actually cleanses their body. Not just her reproductive organ, but her whole body.
I want to read the novel to understand wtf is happening to her menstrual cycle and I’m really glad that the duke is falling for her, I hope I see some cute coupley scenes between them and I WANT TO SEE HIS SON SO BAD TOO