My mom has a Chihuahua too, her name is Lily Rose. Sometimes she yelps when her skin gets pinched, most especially under the arms. We can sometimes move her or pick her up and she yelps. The only thing we can think of is that she got pinched somehow accidentally. We too took her to the vet when that happened, everything checked out fine. She is now 4 years old. They are very delicate creatures and need to be handled a little more carefully than the average dog. As for the shaking, they shake all the time, when cold, excited, scared or nervous. Lily shakes all the time, nothing to be concerned about, just a normal trait of Chihuahaus. Enjoy your little Chi Chi, they are precious. She'll be your baby from here on out.