Backup takes all my iCloud storage?

My iPhone refuses to back up because it says I don't have enough iCloud storage.

I currently have 3.1 GB of 5 GB used; half of this is Docs, half is existing backups. This phone is the only device being backed up.

Question: Am I supposed to delete the current backups to make space for a new backup? Why wouldn't each backup simply overwrite the previous one?

The only things I have, in addition to 1.5 GB of Docs, is a notes app (2.8 KB) and a game app (242 KB). I have removed all photos and music.

I have rebooted my phone numerous times, but couldn't think of anything else to do.

Thanks in advance for any help you can provide ...