Best chance is that you installed an app that doesn't behave like it should.
You can of course check, test and find out what is causing the pain.
That can take a long time.
The best option:
- Make shure to backup everything
- Do a software repair with Xperia Companion
If a software repair fails then charge the battery to >80%, switch off your smartphone and in Xperia Companion after clicking Software Repair tick the : Device cannot be recognised or is a smartwatch.
You'll be surprised how well everything will be restored after the software repair.
Apps that require authentication will ask for authentication again before you can operate tose.
Your device will be lean & clean after the repair and perform better than before.
PS
Chat apps like Signal, WhatsApp etc. require a separate backup from inside the app.
