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I'm happy that you have asked me to assist you with your child and hope that you can benefit from my many years of helping parents like you. I am concerned about the fever going on along with the back pain in that the two could be related. There are some unusual less common infections that can occur in the lower back - one in particular fits her symptoms and that is discitis, an infection of the intervertebral disc itself.
It could be that her fever is coincidental but I'm not sure I'd wait too long to return to the ER or to her doctor tomorrow at the latest. This condition, as well as others related to the disc, vertebra, etc are diagnosed with an MRI, which the ER or her doctor can order if her exam fits that condition, which it sure sounds suspicious for.
In the meantime the muscle relaxer and hydrocodone are fine to keep her comfortable but in any case she needs to be evaluated again regardless by either her doctor, the ER or even an orthopedist. Just tell them you are concerned about discitis ("disk-itis") as it is not that common but can be very dangerous.
Are there any other symptoms that we should be watching out for relating to discitis? Something that might signal us to go to E/R immediately rather than monitor her until going to her pediatrician tomorrow morning?
The most common symptoms are those of a "slipped disc" or herniated disc - pain, radiating sometimes into the buttock or down the leg, difficulty sitting or walking without pain and occasionally, with discitis, pain over the affected disc and of course, fever.
Here is a very brief summary of this infection:
Other problems that can mimic low back injury or infection include referred pain extending from appendicitis, kidney stones, UTI, torsion (twisting) of the ovary, ruptured ovarian cyst, among other less common causes. I would hope that they did a very thorough physical to look at abdominal and pelvic problems, as well as some blood work such as a CBC, Sed Rate, CRP, and a urine culture, even if her urine was clear there.