If you are using WD Sync, and your My Cloud mysteriously is now full, this is a known issue that has been discussed previously. One cause is the versioning feature of WD Sync that saves up to 5 (I think) versions of a file so one can recover a deleted or overwritten file.
One can use the forum search feature, magnifying glass icon upper right, to find the several past discussions on this issue. Here is one past discussion:
