batteries do go bad but not overnight. if its that drastic over the last couple days, its something thats running. You may not have installed a new app but an existing app may have updated and gone rogue

 

Yeah, something is running. Watch dog is a pretty good app, but doesn't really pinpoint the rogue app. Rebooting for me will clear out whatever is draining the battery

 

Download Badass Battery, free, and it works great. Many settings to let you monitor what's using what from CPU usage to screen time, power use etc, you can also see if there's a crazy amount of wakelocks which can drain battery a good amount... really good to help you pinpoint a Rogue app or just to monitor in general and see what's using what.

Battery may need a calibration too, if you were rooted I'd say use Battery Calibrate which rocks when changing/updating Roms to make sure Batt is reporting correct stats. But for stock this is what I did when I got the phone and battery life was quite better vs 1st couple days w/o doing that.
From XDA:
1. Turn off your phone.
2. Fully Charge.
3. When getting 100% signal, unplug quickly.
4. Pull off the battery and put it anywhere.
5. Wait for 180 seconds exactly (be careful )
6. Meanwhile, take this opportunity to dust off the internal compartment.
7. Put back the battery on the right side.
8. Turn on the phone.
9. Pray the Battery god
10. ... and voil