Thank you for the extra information, and giving a better understanding of the situation.
There are a few ways to approach this.
1. Since she was on antibiotics at the nursery, and your vet gave more of the same antibiotic after you adopted her, she may have just become resistant to the meds. This is very common when an antibiotic was not given long enough or in a strong enough mg dosage the first time.
The bacteria does not get completely killed, and comes back stronger than ever before and immune to the medication that was used.
The best thing to do for this is take the cat of all antibiotics for at least 2 weeks, and then try a different medication like Baytril or Clavamox.
The cat may not have a bacterial infection. She have contracted a herpes virus from the other cats in the nursery. Feline herpes is a virus that will come and go the entire life of the cat. It causes all the symptoms you are seeing. The best to deal with it is by strengthening the immune system.
You can do this by using L-Lysine or colloidal silver.
Talk with your vet about Feline Herpes virus. That is a big possibility here. Here is an article about feline herpes virus type 1 (FHV-1).