Almost the same thing happened to me flying AA from ORD to LGA. I checked in 24 hours before, walked up to gate with no checked bags, and when I tried to board, they said my seat had "been dropped." They then proceeded to place me in a middle seat. Only difference is that I am a CX gold (OW Sapphire) so they couldnt care less. Emailed and got a similar response saying I needed to check in at least 30 min before.