My chi is lethargic/ shaking and very quiet. He did eat his meal last night but was whimpering very quietly?

As your Chi is whimpering when he eats, I am wondering if he has a problem with a tooth. That would explain whimpering when eating and the lethargy and quietness. As for shaking, Chihuahuas seem to do this quite often in fairness, at least mine does!

Chihuahua jaws are small (like the dogs themselves) and sometimes there is not enough room for their teeth along the jawline. This can cause problems, so it is well worth checking out.

Generally if a dog is lethargic it is a sign that something isn't right, and a trip to the vet is then a very good idea, just to be on the safe side.

I'm guessing the problem is with his mouth but, as we all know, dental problems can be very painful and distressing, especially for a dog who doesn't understand why he is hurting.