Thanks for that information. There is no specific thing that is readily identifiable. Usually excessive panting can be caused by heart problems, lung problems, fever, or bronchitis. You can read about this here:
Dogs also pant to cool down, but they also pant when they are under stress, in pain, or afraid. There can also be medical causes such as neurological problems, respiratory disorders, and Heart problems such as heartworm, anemia, and fever. She is shivering as well. Shivering is often associated with the same conditions that lead to panting.
I'd go ahead and check her mouth for any teeth issues. A problem may just be developing which is causing her some discomfort but not quite enough to stop her eating. In addition, since they are grazers, you might miss the beginning of a problem with her eating. It is one reason I recommend scheduled feeding times with the food taken up afterward. It lets you know immediately when a dog stops eating. If you see discolored teeth, red gums, or swelling, you will want the vet to take a quick look. Go ahead and put a drop of karo or pancake syrup on her gums to be sure her blood sugar levels remain stable. Low blood sugar can lead to seizures which can manifest as trembling and a dog will pant because they are a llittle scared.
Check her anal glands to be on the safe side. See if they are tender at all. they may not be swollen but still have some discomfort causing the behavior. The last thing might be a disc that is starting to give her problems. It may not be bad enough to interfere with stairs or jumping but still a little achy. Of course a pulled muscle might also be to blame but with no limping, it doesn't sound as likely.
Given her clingy nature, you might want to get a DAP collar or diffuser to help calm her down a little. It may make her a little less dependent on always being on you and more comfortable just hanging around. Monitor her closely and if she develops any other symptoms including loss of appetite, inability to eliminate, limping or she doesn't improve, I'd have her seen.
One other thing you might do is try a little pepcid in case she has a little excess bile from potentially not eating. Read about Pepcid dosages and usage information here:
If she isn't spayed and is close to her heat cycle that might cause these symptoms as well.
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