Hey HN,
after the app I actually used to track my period wanted me to log in and save my data in the cloud, I decided to write my own.
Most apps in this area are based on a subscription model and display far too much information anyway.
For me, a simple calendar is enough where I can add a few notes if necessary.
So that is the result of my work - a simple design and the data is only saved on the smartphone.
(And unfortunately this is also fairly sensitive data in some regions...)