I wrote a period app for my partner, everything is encrypted and never leaves the device.

It has a option where you can set false passwords, so that if you are being compelled to unlock it, it will get up fake data.

Code is open source: https://github.com/cameroncros/PrivatePeriodTracker

https://play.google.com/store/apps/details?id=com.cross.privateperiodtracker

It may not meet your needs, but perhaps worth a look. I am open to adding features (except for ads and tracking obviously).