iPhone shutting down early

Hi,

My iPhone 6 has started to shut down before the indicator has reached a low level. For instance today it shut down with 16% showing. When i tried to restart, it told me to plug it in. Even though it was switched off it still counted my steps for the Health app!!!!

Do i have a hardware issue? What should be my next move? Full reset and if that doesn't work then take it in to Apple?

Thanks for any input.