Some period tracking apps will share your data. Stay safe, use one from @fdroidorg without Internet permission.

Possible options are:
drip (safest!)
Periodical
log28

All of these apps don't ask for internet permission and thus can't share your data. Drip goes the extra mile and explicitly opts out of Android's backup framework so your data won't be backed up on Google servers.

Please boost for those who need it!