Why does my iPhone turn off when unplugged even when it says its full

I bought a brand new iPhone 7 last week from the apple store, it has worked fine for the week I've had it. It was charging over night and this morning i went to use it and it just turned off. I tried to hold the power button down to turn it on and the apple logo came but then went away. I plug it in and it says it is at 1%. I unplug it and it jumps from 1% to 100%!!! What is going on, whenever i unplug it after about 10 seconds it will turn off again and then plug it in and it goes to 1% but then it goes to 100%!!!! Very annoyed because it is brand new