I had to stop nursing my daughter at 11 weeks so I could start chemo. I nursed my son for a full year despite being a full-time working mom. It was a n easy decision - my baby needed me more than my milk. She did fine on formula, and your daughter will, too. The transition will likely be a lot easier than you can imagine right now. Just be patient as she gets accustomed to different nipples, etc.
Having worked in the pharmaceutical industry for the past decade, please do NOT nurse her if the physician/pharmacist are telling you that the medication is not safe during lactation.
Most drugs are not even tested in nursing mothers because of the possible implications. Most are tested on nursing animals (usually rats and mice), and if the active ingredient is detected in their milk, the researchers will determine the likelihood of it crossing into human milk. As a biologist and a mom, I'd not be willing to take the risk with a child that age, especially with an antibiotic to treat a kidney infection.
Best wishes to you. In our case, it was a no brainer - it sounds like it's the same for you. I hope your baby transitions as easily as ours did.
Depending on how long you have to be on the medication, you can pump and dump until it's out of your system and resume nursing. I had to do that with the imagine contrast for my PET/CT scan and while taking Naproxen for an injury from a car accident with my first child.
Good luck.