Hello. Thanks for the question.
Trembling can be due to one of many things- pain, anxiety, fever, hypothermia, stress, ect.
The most common cause in a middle aged boston, would be pain. Back pain to be more specific. This is very common in small breed dogs and brachycephalic dogs. They often present with trembling, reluctance to jump up, reluctance to climb stairs, an arched back and occasional yelping when picked up. They don't have all of those signs, but some type of combination.
The treatment is typically with anti-inflammatory meds (Rimadyl), pain meds (Gabapentin, Tramadol) and crate confinement for 1-2 weeks.
I would keep her confined tonight and then have your vet palpate her tomorrow. If it's back pain, they can start meds at that time and it should clear up nicely.
I hope this helps, let me know if you have any other questions.