Why is iCloud telling me it's almost full when I've never used it?

Suddenly I'm getting notifications that my iCloud backup is almost full. Since I don't use iCloud to back up anything (I live where internet is unreliable) I don't know what's changed... HELP!

I have a hard drive backup since I can't rely on my internet access, and my MacBook Air should only be backing up to my external drive. I don't use a lot of data, and the only things that should be synced via a central location like iCloud are my contacts, calendar and messages. This has always worked for me for years.

Have tried to delete any random things I could find in iCloud, but it doesn't seem to affect the massive (for me) amount of files showing - messages (2.57 GB) and backup (1.8 GB).

Is there some automatic backup that's occurring that I can't identify? If so, how do I turn it off? I'm not very tech-savvy (hey, I'm almost 75 and have some residual brain damage from an accident) so I'd really appreciate a simplified answer if at all possible.

And since I'm on a fixed income, paying for more storage isn't viable...

Thanks for any help you can offer.