Why Is My Dog Panting?

He either has developed negative association with the bed for some reason... Or is ill and those are the times that you are more closely observing him and noticing the symptoms. 

Are you the only one he has contact with... Or is it possible that someone else doesn't want him on the beds, and has made that VERY clear to him when you weren't around... Or was there a bad thunderstorm of some other unusual event last week that scared him while he was in the bed?  Sounds to me like a fear response... But it's possible that he's ill and the added attention/excitement takes more energy and so heightens his symptoms.