Medicine side effects

I adopted my sweet Ally a few weeks ago, when I noticed she had bad diarrhea. She tested positive for roundworms and giardia. I decided to leave her at the vet for eight days so my other cats wouldn't get contaminated. We she cam back home, she was acting lethargic, and her back legs seem weak. I bought her back to the vet, who said maybe its and allergic reaction to medicine. Has anyone have this problem. If so, how long before they went back to normal?

Can you list all the medications she was on?

I've never seen a cat lethargic from Giardia meds, and its odd that they didn't notice this there.  Was she eating ok at the vet?