Dog Heartworm Treatment

I got a 12 week old puppy from the human society yesterday and they gave him a parvo shot along with his other normal puppy shots (?) and a heart worm shot. We got him home and the first hour or so he was very playful and energetic but after that he slept the rest of the day. He only ate and drank very little. During the night he threw up 2-3 times and has no other symptoms. This morning we to him to the Vet and they did a feces parvo test. It came back positive. We took him back to the humane society but they told us the parvo shot can cause the (false) positive test results. I called another animal clinic, for a second opinion, and they agreed with the humane society in that it could be just the shots effecting him and that a false positive on the parvo test is likely.
I have searched the internet and I can’t find any information on the false positive after the shots. If anybody has any information please let me know. For now we are keeping him for a couple of days to see how he feels or acts. I really want to save this dog but don’t have the money for parvo treatment.
Thanks for your help.