iPhone 8 Plus battery suddenly draining too fast

I have a problem with the battery draining too quickly on my iPhone 8 Plus (current iOS is 11.3). If I recharge it to 100% and then leave it overnight while I sleep, the battery is already down 15% or more. Even on Airplane Mode overnight it does this. It didn't use to do this before until within the last week or so (no new apps or behavior either). I have Background App Refresh off, have drained the battery down to 0% and recharged to 100% to recalibrate, and have even factory restored my phone but it still drains fast despite all this. Battery Health says the battery is at 97% so I'm not sure what else I can do or check to resolve this problem?