For the past 3 days my Chihuahua has been shaking, doesn't want to eat or drink seems to isolate himself. . When I try to pick him up, he barked as if something is hurting him.. . The shaking part really scares me. . he seems to be so helpless when he's very playful. .

Go to the animal shelter and ask if they can help since its qite posebly Animal worm innfection

Animal shelters are not, generally, vet hospitals. Animal shelters usually only care for abandoned pets.

Unfortunately those symptoms can be an indication of many problems. If he is yelping when you touch his stomach and doesn't want to take in food or water then it's possible he has a bowel obstruction.

My advice, get him to the vet as soon as possible.

J

I agree, vet asap. Too many possibilities.