I assume that people who need to know this would already know it, but I am not such a person and did not know it, so I will pass this on from a correspondent who wishes to remain nameless: There's a FOSS menstrual cycle and fertility tracking app named "drip." created by feminist female developers with privacy at its foundation. I'm not qualified to opine about its safety or security but developers say everything stays on your device: https://f-droid.org/packages/com.drip/