The problem is that there is literally no other free and open-source app to track your periods, so you're forced to use some proprietary piece of shit that sells all possible medical information about its users.
There seem to be lots of FOSS period tracking apps available, look at the other comments in this submission!
What seems to be lacking, is a FOSS period-tracking app that also lets you share stuff with a partner, which is the reason me and my partner use Flo in the first place.
What seems to be lacking, is a FOSS period-tracking app that also lets you share stuff with a partner, which is the reason me and my partner use Flo in the first place.