icloud backup

My iCloud storage on my iPhone 7 shows 3.2GB OF 5GB but suddenly my iCloud backups are failing because it says. ‘This phone cannot be backed up because there is not enough iCloud storage available’

It backed up 2 days ago and the storage has not has not changed from 3.2GB in last 2 days.

I know I can purchase more storage, but would like to understand why this has started happening.