Why is my iPhone 6 battery all of a sudden draining really quickly?

About a week ago I noticed my iPhone battery would drain very fast even if I haven't used it. I'm up to date with the newest iOS and everything else seems to be normal. Any ideas what could be wrong?

You've downloaded a rogue app. Poorly-written apps can suck batteries dry if they run all the time in the background.

I had the same problem happen and i had to take it to my cellular provider to have them replace the phone completely

Could be a battery that has seen better days