I spoke with the rewards line this morning and the agent fixed the reservation in 30 seconds. According to him and the reservations agent, the property made the change on the reservation. I asked why they might do something like this and he related that if they were processing the reservation and saw the cert on my account, they took that one instead of using my points. He corrected it and now all is well.

No big deal in the end. I guess the moral of the story is to always keep an eye on your reservations.