This happened to me as well. It is an issue with the most recent update and they're aware of the issue. After spending an hour on the phone with apple support (where they tried to insist it was a hardware problem) they eventually instructed me to get a genius bar appointment. WHICH THERE AREN'T ANY AVAILABLE FOR AT LEAST A WEEK (which is a huge problem in my opinion). So I had to go yesterday and try to get a walk in. 4 hours later I walked out with a new phone.
unfortunately they said the engineers are aware of the problem but there isn't a new update and you're going to have to replace your phone. But you won't pay for it at least.