I’m not really knowledgeable enough to ever feel confident about analysing whether apps are truly privacy respecting. So, I’d suggest look into these for yourself and see what you think.

I’m using Mooneva for a period tracker that helps analyse signs and symptoms at different points in your cycle. https://f-droid.org/packages/com.mooneva.app

Someone else mentioned MediLog. It can be used for lots of stuff. I’m just using it to monitor my weight because I don’t need the other pieces at this point. https://f-droid.org/packages/com.zell_mbc.medilog.fdroid

And when my body is being weird and I want to look for correlation with my symptoms and my diet, I use Life Notes. https://play.google.com/store/apps/details?id=com.sebferrer.life_notes