I would definitely be disappointed but seeing as you were charged a lower rate I don't think you can do anything other than cancel and get your money back. I would let your online agent and agency know that you are not happy about this and tell them that you booked an A and you feel you should get an A. I take it that this is a Holland America ship. One consolation is that the Cat A and Cat B cabins on most HAL ships are identical except for the deck. As Lisa said you still have a good chance of getting a bump up to an A cabin.
Take care and don't let this ruin your trip. Take the extra money and have a couple of cocktails or spend it on a trinket or two.