Thank you for your question. My name is***** I have been a small animal veterinarian for 14 years and I'd like to help with you question.
It sounds like your dog has multiple swollen lymph nodes.... the number one concern with multiple swollen lymph nodes is definitely cancer. It sounds like your dog was tested and it was negative for cancer. If the test was done with a needle aspirate then I might repeat the test with a biopsy just to be sure.
Other than cancer, multiple lymph node enlargement is usually from a systemic infection. In some areas fungal infections can be a cause. I would check with your veterinarian to see if you live in an area where fungal infections are possible. If not, then other systemic infections include bacterial, heart worm, lyme disease. Your veterinarian will need to run more tests to try to find the problem.
In the mean time, your dog should be in the hospital on IV fluids and antibiotics. Has your veterinarian recommended hospitalization? If not, I would ask him about it. If they are not able to hospitalize your dog then they can likely recommend some place that can do it.
I hope that helps, but if you have more questions please let me know. If not, please dont' forget to rate my answer.