iPhone 6 with a lot of issues

My iPhone 6 is recently having a lot of issues. It heats up extremely and in 6 hours time goes from 100% battery to 0. Even when I'm not using it. Also the screen doesn't want to rotate anymore. In the Messages and Mail apps I get a lot of messages saying 'Undo typing?' 'Redo typing?' 'Redo trash'. In FaceTime the camera switches from back to front without warning. I went to a repair shop where they told me first to change my charger. When this didn't help they changed the battery of my iPhone. Does anyone have an idea to what this is due? Personally I wondered if it could be attributed to the fact that I dropped my phone quiet a lot or if I have a virus or if it's bugs in the iOS. Thanks for your help.