There are a lot of apps out there to track your period, your mental health ups and downs, and more. But the simplest way to keep track of what’s going on with your body is just to create a calendar (in Google Calendar or wherever you keep your appointments) called “health.”
Then just create an all-day event for anything you want to note: “Allergies are bad today” or “sore throat” or “diarrhea” or “period started.” Feel free to come up with code words if you’re concerned with people seeing the calendar.
You can add notes in the “description” field, and you can get a big-picture look at what happened with the Schedule (used to be Agenda) view that just gives a list of dates and events. You can also search calendar events, giving an easy way to answer when your doctor says “How long ago did this start?”