iPhone hack

nah, more likely that the battery is dying

1. Open the Settings app.
2. Scroll down and select Battery.
3. Tap Battery Health, where you'll find a report detailing your battery's health percentage, and any suggested steps you can take to improve performance.

You have a very old device and it may be time to replace the battery.

PutneyWolf wrote:

I understand this sort of battery behaviour can indicate the phone has been hacked. Is this likely?

That would be completely false.