As a thank you for being a former MobileMe member, you received a 20 GB complimentary storage upgrade when you moved to iCloud. Your upgrade expires on September 30, 2013.
When it expires, your iCloud storage will be automatically adjusted to the free 5 GB plan. Note that you are currently using 5.99 GB of storage. If you exceed your storage plan on September 30, 2013, iCloud Backup, Documents in the Cloud, and iCloud Mail will temporarily stop working.
To continue using these iCloud features without interruption, reduce the amount of iCloud storage you are using or purchase a storage plan by September 30, 2013.
For more information, see this article.
The iCloud Team