iPhone 6 plus charging problem

Hey, so recently my phone was on 3% battery so I plugged in the charger but nothing happened, tried it again but nothing so I turned it off and on. When it turned on it was on 15%(insert x-files theme song) and then suddenly shut itself down. I plugged my charger in again and waited for like an hour, it said it was charging but when I turned it on it was on 8% and almost immediatly shut itself down again, then few minutes later the same happened. Now no matter how long I press the lock button or the home + lock to hard reset nothing happens, only the charging sign is there on the screen. Any idea what this is about?