A dog, usually lively, who suddenly hides under the furniture, refuses to play, to eat and seems downhearted, must be reminiscent of chocolate poisoning, especially during the holiday season. 

The theobromine contained in chocolate, belongs to the group of methylxanthines, an alkaloid of plant origin which stimulates the central nervous system. Extreme agitation can therefore also occur, with confusion and even aggression. 

Any change in behavior in general should suggest poisoning or intoxication.