A quick search showed me these two: https://github.com/arnowelzel/periodical https://github.com/arnowelzel/periodical and https://github.com/wildeyedskies/log28

A quick search showed me these two: https://github.com/arnowelzel/periodical https://github.com/arnowelzel/periodical and https://github.com/wildeyedskies/log28 https://github.com/wildeyedskies/log28. Both are available on F-Droid. I don't use Android and I don't have periods, so unfortunately I can't really tell how decent they are - but they both look somewhat reasonable on screenshots.