Giardia positive, please help!

We live in a big city and my 6.5 month old puppy got Giardia. We were shocked because I am a freak when it comes to hygiene. The vet said it was extremely common in our city and did not seem too concerned at all. She sent us home with two meds and minimal information. My puppy now refuses to go on walks. She has become more and more lethargic starting since the second day of meds. Her stools are normal and her appetite is the same but she does not want to walk. What might be happening here?