Hi, sorry to hear Merleau is unwell again
The fever shouldn't be from the treatment no, normally a reaction is 24/48 hours after anything given so should have shown by now if its going to.
I think you may need to re test, the fever is happening because there is some kind of bacteria/virus that has not been 'fixed' by the treatment given. The shivering is just a symptom of the fever but am pleased you are back off to the vets this morning.
I think further examination and tests is what Merleau needs. Please do let us know an update x