I might not need the battery replacement after all (not yet anyway). Turned the phone off and fully charged the phone. It's definitely back to before because it only lost 10% overnight (9pm-7am) and hasn't been depleting like yesterday. Usage and Standby times are more accurate. They used to be the exact same lengths (where Usage was freakishly long), now they're more consistent with my actual phone use.
No idea what just happened with my phone.