Traditionally, it's been thought that pet anxiety disorders were always connected to a traumatizing, fearful event - being separated from your owner, hearing loud noises etc.. But this doesn't explain why some dogs and cats seem to be constantly anxious and fearful, even when they're not in a stressful situation.
Some veterinarians now suggest that dogs and cats can have Generalized Anxiety Disorder (GAD).
In humans, Generalized Anxiety Disorder (GAD) is diagnosed (DSM-5) when a person experiences 6 months or more of chronic, exaggerated worry and tension, even though nothing seems to provoke it.
For pets, GAD should be considered if chronic anxiety lasts 2 - 3 months after, or without, exposure to stressful stimuli.