Hi katiekatiekatie9797,
You mentioned you weren't sharing any photos or albums, are you a member of an iCloud Shared album on another device using the same iCloud account that you use on your Mac? Another user may have met the shared limit as some of the limits are combined.
You can learn more about those limits to see if any apply to you here:
My Photo Stream and Shared Albums limits - Apple Support
If you're not able to find any instances of you or other users meeting a shared album limit, we encourage you to do a little research to see if someone may have access to your device(s) or account. These instructions will show you how to determine if any other devices have connected with your account.
Check your Apple ID device list to find where you're signed in - Apple Support
Use your Mac to find where you're signed in
1. Choose Apple menu > System Settings. In macOS Monterey or earlier, choose Apple menu > System Preferences.
2. Click your name. In macOS Montery or earlier, click Apple ID.

3. Click any device name to view that device's information, such as the device model, serial number, and OS version. The information might vary depending on the device type. You might need to answer your security questions to view device details.
If you see a device you do not recognize, remove it from your account and take the steps recommended by Apple to recover and secure your account.
If you think your Apple ID has been compromised - Apple Support
Thank you for being part of the Apple Support Communities.
Kindest Regards.