Q. My 4 year old Chihuahua mix began having a series shaking/panting episodes (last 15m- 1hr) out of the blue. Vet's tests say its not physical.
A. There are many causes for shaking/panting. The shaking and panting are both signs of stress, and your dog may be dealing with anxiety, or stress, related to an event that happened, or is happening. I realize you cannot answer questions on this, however, I will ask some questions that you can ask yourself. Have you recently moved? Have you ever hit or yelled at your dog? Has the weather been bad lately (storms)? Have you had any new guests stay over recently? Have you had any dogs come to your home recently? Have you had any dogs or cats in your yard recently? Was your dog frightened by something initially (a falling pot/pan; a loud bang from the washing machine; a gunshot; a backfiring car/truck; someone screaming in your home/a fight)?
If you answered yes to any of those questions, it could definitely be that. Dogs don't typically hang on to something for very long, but if it really frightened your pup, then she/he could be feeling serious anxiety related to that specific event, and relating other events to that one.
Do not yell, or hit your dog. I'm not assuming you do, but if you do, please stop doing that right away. It could be that your dog is afraid of you specifically, and you notice the shaking/panting when you are near, because that is the only time your dog is doing it.
If you'd like to purchase a consultation with me (I know, it's a lot to ask, but I really feel like I could help) I'd be more than happy to ask you many questions, and together we can figure out what the heck is going on here. It's important that your dog is comfortable, and if your pup is always feeling anxious/uneasy, then his/her quality of life is in jeopardy.