Unusually high battery drain is a symptom, not a problem. There are many possible causes for this symptom. In theory, as many as there are apps installed on the phone.
Sudden change in battery drain is caused by an app or apps that are using more energy than they should. It can happen at any time, but is slightly more likely after an update, because the update process interrupts apps in the middle of some activity, either foreground or background, and some apps do not recover cleanly and get stuck attempting to repeat a failed data update, essentially forever. Killing the app doesn't always resolve it, because the real problem is data that was corrupted by the interruption, and the app will continue trying to recover the corrupted data after it restarts.
MS Exchange email accounts will do this frequently, due to a bug in the Microsoft ActiveSync process which, unfortunately can never be fixed, because it would break millions (billions) of older MS Exchange clients that cannot be updated. If you have an MS Exchange account the only fix is to delete the account, restart the phone, and add it back. Or have your administrator restart the MS Exchange server (which most admins don't really want to do). If you are interested I can provide technical details on this bug, but I don't want to distract from solving the problem.
Other IMAP email accounts can exhibit a similar problem; again, the easiest solution is to delete the account, then add it back. As the email server is always the master for IMAP accounts no messages will be lost.
Accounts that sync data from a server (and this includes iCloud) can get out of sync between the server and the iPhone. Given enough time this can resolve itself (at least for iCloud), but it's easier to shut down the sync service, restart the phone, and start it up again. Yes, even for iCloud syncing. And I had to do this over the weekend when my iCloud Contacts stopped syncing correctly. It took about 14 hours for my contacts to again sync.
As many 3rd party apps sync with a server all can be suspect. That's why checking to see which apps are using the most energy in Settings/Battery is a good diagnostic step.
IOS 11.2.5 battery drain
