Check the iCloud settings on your iPhone!
Apple recently release iOS 10.3 and a bug may have caused some iCloud services that had been previously disabled to be mistakenly re-enabled.
To check which iCloud services are enabled on your iOS device, open the Settings app and scroll down to the "iCloud" section. A list of apps and services using iCloud is front and center, and anything that was enabled via iOS 10.3 can be turned off using the toggle buttons.
There quite a few apps and services use Apple's iCloud feature, including Photos, Mail, Contacts, Calendars, Reminders, Notes, Safari, News, and more.