iPhone 6 visual noise on the screen followed by a red triangle with exclamation mark

It happened while charging and using iPhone today. First there was a short weird noise from the phone, then for a half a second I saw some slight visual noise overlayed over the screen image, it looked like a bad TV signal. And immediately after that, a red triangle appeared with exclamation mark, also for a moment. After that it worked as usual but I turned it off and turned on after a few hours. I used it for a couple of hours and it happened again, but this time there was no sound, and it wasn't charging at the moment. It looks very unusual, sci-fi like experience.

What can it be? Should I really worry? Can it be something like short circuit, can it kill me like there were cases when people were killed while charging their phones?

I have to add, over the past weeks it warned me a couple of times during charging that I use non Apple accessory (cable I suppose), but i actually use Apple's cable and charger.

iPhone 6 Plus, 64 Gb