Sick Naso Tang, Help!

[KPG007]

I was feeding my tank today and noticed my Blonde Naso Tang not eating, in fact it was swimming rather calmly around the tank (instead of with a bit of purpose as it usually does). I've had the Tang for about 1 1/2 years and it has always been the picture of health. Yesterday it was swimming well, grazing as usual and very anxious to eat as usual. Eyes are bright and clear, body is smooth with no signs of damage or parasites, muscular with a nice little happy belly. All the other fish in the tank are behaving normally.

Is this normal, and something that happens from time to time or is it sick? Is it something that will pass or is it on the way out?

Help, Please!
Kelly