Battery drain in iPhone 6

The problem started when I first downloaded iOS 11 back in October. My battery was draining up to 20% in minutes, but after that, my phone stayed on that level for another few hours. It happened several times but then the problem disappeared and everything was alright. Yesterday this whole situation started happening again (this time the battery drains from for example 70% to 12% and doesn't stop), to such an extent that when I'm using my phone - the screen starts getting somehow frozen and decides to restart the whole device itself. After the phone is restarted, the percentage of the battery loads back to 70% but then it goes down again. I don't know what's happening, everything was good, I never had any problems with my phone, my screen is always dark, the battery is always on that power saving mode, I don't have open applications in the background... My iPhone is updated to the lastest version of iOS. I don't know what's the problem, I have never had any problems with my phone, the battery started getting weaker in October, as I've said, then everything was great, but now it happens again. I've been using this phone since 2015.