I am sorry to hear that you lilttle dog is not feeling well.
Polyflex is an injectable ampicillin, and the most common side effect is pain at the injection site.
I am worried that more than kennel cough is going on with your dog. Kennel cough ususally doesn't result in inappatence and lethargy. Your dog should be pretty normal other than a hacking cough.
It is possible that the kennel cough has progressed to pneumonia, which would be consistent with some of the signs you describe.
I think you should take your dog to someplace that has 24 hour care. She could probably benefit from IV fluids and nutritional support. She may need more test to determine what is going on with her.
If you like, I am happy to help you find a referral center in your area.
Best of luck and let me know how I can help.
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