My 17 year old yorkie shaked throughout her entire life whenever we were out in public and people gave attention or when we'd have visitors. I see lots of small dogs do this so I just assumed it was a small dog nervous reaction. Anyways Kecia was healthy and lived a long life with her shakes. My new pup Pippa did it the other day when we were out and about and a stranger came to pat her...I'm thinking it's not that uncommon...but I'm no expert
Everyone would always ask if she was cold but as soon as they left she would stop.