Our pup is so sick, we've been to the vet 3 times in the past 2 days. Do you have any ideas of what could be wrong?

I have a new lab puppy that is sick with diarrhea and vomiting. I've taken her to the vet 3 times in the last 2 days. She has had parvo and parasites tests. All came back negative. The vet gave her a shot for vomiting and two different injections under the skin to rehydrate her. She's 8 weeks old, but I got her at 6 weeks. The breeder told my husband and I that she was 7 weeks old and that she could eat normal puppy food, so we immediately began feeding her Purina Pro Plan puppy food for large breed dogs. Now she won't eat anything, even the vet's special food, rice or other things we've tried. She won't drink water either. Her temperature is normal. She had all of her first shots and deworming by the breeder's vet at 6 weeks, but our vet gave her an additional dewormer anyway, which did not produce anything.

The vet wants us to come back and spend another $200 for x-rays and blood tests. We've already spent $250 for the last 3 vet visits and just can't afford more expense like this. I looked online and saw your email addresses. Please help. We've grown very attached to this puppy and want her to with us for a long time.

Denise