Hi, I had that problem last week. In my situation I had a faulty sd card. But my phone was also switching itself off when I stopped using it. I looked online and the answers was that some app may be causing this kind of issue. I would suggest to try check if you are having the same issue in safe mode. I had andas Sony technical team told me as a next step to remove sim card and check if the problem remains and finally do the same with SD card.
Hope that helps a little.
