Did you book a Guaranteed Category, or a Cabin Assignment? This does sound odd either way. Usually if it is an assigned cabin they won't change it unless the cabin is needed for some special consideration. Say wheelchair access or a connecting door.
If it was a Guaranteed Category it would be extremely rare for them to lower the category. And they would contact you and make a rate adjustment.
This is indeed strange. I have had miss tagged luggage before and it found it's way to the cabin. But what a hassle and worry! I wonder what others have to say about this.
I hope you get this straightened out!