Chihuahua Puppy 8 weeks old won't eat?

[saturn]

I got a chihuahua puppy on Sunday, on Monday we took to the Vet for puppy check up. They said he was fine and asked if they could give him the heart worm by mouth med I said OK. On Tuesday the puppy started to trough up and did not want to eat, that morning he was fine. Took him to Vet on Wednesday. Was told he was dehydrated and that he has a virus, possible gastrointeritis. They gave him fluids and they staated he eat for them. Brought him home with antiobotis he also ate that evening. On today Thursday he would not eat or drink any food. They have him know in the hospital. Can someone tell what could be happing to him. I've asked the vet could it be Parvo and they say that not likely. But I'm not sure. HELP we have grown so attached to him. I thought it could also be the heart worm med they gave him since he was fine before his visit.