Last week, while the outside temperature was about 20 Celsius, I entered a cold room ( -6 Celcius) for about 20 minutes.
While in the room, my iPhone 6 suddenly shut down. I was not able to turn it on until about 20 minutes after getting out of the cold room (and charging it).
Ever since, my battery has been acting up. Dropping suddenly from 50% to 10%, or shutting down at 30%.