if she's getting back pain, you need to get her back in to hospital now to make sure it's not a kidney infection (which it what happens to untreated UTIs).
my 2.5-year-old has vesicoureteral reflux, which is a condition in which the valves at the top of the tubes between the kidneys and bladder don't function properly, and urine flows back up into the kidneys. this means any UTI has a greater chance of developing into a kidney infection. kidney infections leave permanent, irreparable damage.
she was diagnosed at four months old via a micturating cystogram (catheter inserted into the bladder & dye injected under an x-ray, visible backflow of urine = positive diagnosis). the damage to the kidneys is continually monitored via ultrasound and DMSA scans (radioactive dye injected into cannula in hand, bloodstream carries to kidneys which are then viewed via MRI).
she has permanent damage to the right kidney as a result of two serious infections at three and four months of age.
permanent kidney damage means a predisposition to high blood pressure, so we have to watch her sodium intake quite carefully to keep this in check.
we have lost track of the number of times we've been in hospital. every time she gets a fever, in we go to rule out another UTI. it's extremely stressful for us as parents and for her as a scared little girl, but we have to protect her kidneys.
Please take your stepdaughter into the paediatric a&e and ask them to rule out VUR, for the sake of her little kidneys. she could have serious damage that has never been discovered.