Why might my dog's left arm/paw be shaking?

I am not a veterinarian but I guess your dog must be suffering some neurological disease or he/she might be having a heart problem.

I am inclined to think about a neurological disease...please ask to your vet.

Sorry I cannot help you any further...

Most likely she has hurt her leg, check her paw for any cute or thorns in it would be the most likely thing! And if she has hurt her leg not her paw then just leave her for the day and it should get better by tomorrow. My dog does it sometimes, shes normally better within the hour. If it is more serious than just a 'bashed' leg if you will then get her checked out, but I wouldn't worry about it my dog does it a lot. Its leg would tremble because its holding it up for a while, which is straining on your muscles, even humans do it.

What kind of dog is she? With labs and other dogs with ears that fold over they can get ear infections really easily. We put peroxide on a cotton swab and swab out our lab's ears a couple times a week and ALWAYS after swimming. The smell alone doesn’t necessarily mean anything, even with this frequent cleaning our pup still has a light smell in his ears sometimes, but the itchy thing may mean she has an infection. I would get it checked out to be safe and after that start a regular ear cleaning regime to help avoid this problem in the future.

Should be fine. Might have accidentally had some stimulants that made her sugar levels too high :'). If it carries on tomorrow, go to a vet.

is she nervous or anxious

arthritis.