Why is my dog shaking and panting after pain meds and antibiotics?

Poor Winnie!
Try not to panic at this point, sometimes dogs can have odd reactions to medication. It really depends what sort of pain killer, if it is some sort of opioid such as buprenorphine/methadone etc then shaking is very commonly seen as it enters the nervous system. However with other painkillers such as metacam etc then it wouldn't be so common.
Secondly, a dog can always have an allergic reaction to any medication so if this is a medication she has never had before I would consider calling your vet for advice if her condition does not improve.
Finally, there is always a possibility that something else is going on. I would double check that you gave the correct dose of medicine, and if not and you overdosed ring your vet immediately for advice or to be seen. If she continues to deteriorate I would take her into the vet in case she is having a medication reaction or it could be that her original condition is getting worse or developing.

But most importantly try not to panic for now, monitor her in a calm and comfy environment. Shaking can be incredibly common with many pain killers and they can also cause drooling and odd behaviour. If she becomes sick or starts to loose consciousness please go to the vet.

If you want to post again with the name of the medications and dosages I can advise you better as to their specific side effects.