What Causes Lower Back Pain With Pain In Legs?

My daughter (24) was diagnosed with a bladder infection 4 days ago and has been on antibiotic. A fever of over 101 has developed, as well as lower back pain and pain in both of her legs. She returned to the doctor this evening, was diagnosed with a kidney infection and given a different antibiotic. Doc said to go to the hospital if it gets worse. This sounds alarming to me. Should she go to the hospital regardless?