Very good question. I believe you would be correct though I'm very tired so a second opinion just to confirm would be nice.
In Duo's sentence we are talking about a characteristic that the person normally displays so 'ser' is correctly used.
In your sentence however we are saying it is a condition that the person is displaying for today and not necessarily something they normally display. 'Estar' is used for these types of situations.
Edit I am now no longer tired and after reviewing my earlier statement would now like give a second opinion: It is correct. :)
Alvera13. I'm sensing some frustration from you but try not to give up. If you believe you had the right answer you do have the option of reporting it (use the REPORT button at the bottom). If you have questions as to why your answer was marked wrong then your best option would be to copy and paste it into a post on this discussion board and ask other users what the error is. Also, remember that if you REPLY to another users post that user will be notified directly and might be more likely to respond.
Also, you can delete your extraneous posts by clicking the delete button. This helps keep the boards clean so it is easier for us to help one another. Thanks and good luck.