The pain in your back could very well still be due to a kidney infection causing costovertebral(CV) tenderness and pain. Make sure you take the prescribed antibiotic as prescribed and also drink plenty water. If the pain worsens with your following your doctor's prescribed medication directions further on today, call your doctor and discuss your option of going tonight to a local emergency room taking your medication with you for a provider there to see and they can test you there in the emergency room for kidney stones by doing a CT scan and letting you know if one or more is seen on the scan. Then depending on what is noted in the emergency room, you can follow-up with your regular doctor after that emergency room visit. The emergency room physician or your own physician may continue the antibiotic you are on or he or she may decide to change the antibiotic with the addition of something for pain as needed.