

ITT people mentioning bipolar disorder but I would suggest a strong onset of Premenstrual Syndrome (PMS).
My only question to you is, when did your menstruations start this month? You don’t have to reply to this.
But look up whether they started the day you were sad, or one day/two days prior.
If it was the case, it is very likely PMS.
Keep a calendar of your cycles and you will know.
Causes of PMS may be one or many, like birth control, undiagnosed depression, a stressful time, etc. etc.
Edit: typo
I’ve been looking up a solution myself for years. I ended up with encrypting my note folder with Gocryptfs and synchronize the folder with Syncthing.
Requires quite the setup at first, but perfectly fits my needs (I also use extensively the terminal).
On android there is DroidFS to decrypt your synched folder and read the notes in mobile, but I couldn’t find a way to edit them.