Yup, exactly. Nothing digital is truly safe. You want to properly avoid having your data secretly subpoenaed? Use a fucking journal or planner, and just mark your period days with a different colored pen, or put a raindrop in the corner and say it was days you personally predicted it to rain. Turns out, your tarot cards are just really bad at predicting the weather.

You could be open about it, and use it to track symptoms, calculate days between cycles, etc… But the more detailed you make it, the more obvious it will be if someone else gets ahold of it.