Constant 'iCloud full' messages, despite having 3.4GB available

Since recently changing my primary icloud email address, I constantly receive alert messages on my iphone 6 and ipad air 2 that the devices are unable to back up, because 'iCloud is full'. But when I check in settings (on both devices), it appears 3.4GB is available.

I don’t back up photos, and don’t think I backup anything else that takes up considerable space. And I have photo stream disabled on both devices. Any ideas as to why I still receive these messages (every **** time I charge!) appreciated. Many thanks