iCloud storage is full but it's not - how to fix

I got a Macbook Air M1 a few months ago. I don't have an iPhone, iPad, or other device accessing iCloud. Some time ago I started getting Finder messages that my. iCloud storage is full. I have the 5Gb "free" option. When I look in SysPrefs, AppleID, iCloud, it says I have 4.68Gb available. Nothing I have takes up much room. I have downloaded .mp4 files and photos but I always Trash them when I'm done with them and they don't appear anywhere I can see. Plus they don't show as taking space on iCloud. Any ideas how I can fix this and get the use of that other 4.68Gb of space?