Fee for iCloud storage

You can see your iCloud storage in Settings>Your Name>iCloud>Manage Storage. If you have used all of the storage in iCloud, you are going to get a message that the storage is full. You pay a monthly fee for a fixed amount of storage. The amount of storage never changes. If you pay for 50GB of storage, once you fill it up, you either have to delete content or move up to the next storage plan. That’s what you pay for.

Think of it like renting a storage space. You pay a monthly fee to rent a fixed, total amount of square footage. The amount of space that you pay for doesn’t increase each month, but you still pay the same fee every month for that fixed amount of storage. If you need more space, you have to rent a bigger storage pod.

This might help explain things a little better.

About storage on your device and in iCloud - Apple Support

iCloud only managed iCloud storage such as iCloud Drive files and photos. There are some other types of files that are not in iCloud. If your device fills up, then iCloud will not work properly.

Go to Settings > General > iPhone storage to review your storage.

There was a new OS update recently. If you have those set to automatically download, then that could be the problem. Either install the update or delete it. You could also have some apps that do not use iCloud that are taking a lot of your storage.