If you don't happen to use all the resources / services of the iCloud (Notes, Reminders, Calendar or some other) and unchecked any of these options in your device's Settings (be it an iPad, iPhone and / or iPod touch), good to check your account.
This is because, as informed by the MacRumors, Apple found that the iOS 10.3 there was a small bug that could improperly activate one of those options that the user deselected in the settings.
Here is the full email that Apple sent to some users:
We discovered a bug in the recent software update, iOS 10.3, which impacted a small number of iCloud users. This may have inadvertently reactivated some iCloud services that you had previously disabled on your device.
We suggest that you see the iCloud settings on your iOS device to ensure that only the services you want to use are enabled. Learn more about how to manage your iCloud settings or contact AppleCare if you have any questions.
To see if everything is the way you would like, go to Settings, then tap on the new area (the first) at the top (with your name and avatar). Then go to iCloud and see if the services selected are really the ones of your choice.
Also according to Apple, the iCloud Photo Library, the Keys and the Find iPhone were not affected by the problem. The bug has already been fixed in iOS 10.3.1, so anyone who just skips iOS 10.3 will not suffer from the problem.