My daughter has this recurrent issue where she has the symptoms of a UTI, but the tests and cultures always turn up negative. Any ideas?

She has a low-grade fever off and on. Urinary urgency, urinary frequency, sometimes painful urination and accidents. She complains of back or side pain at times. For a while she also complained of joint pain (most often knees). She clearly feels bad some days, barely playing and saying she feels tired and cold. This went on for about a month earlier this year, and now it's back.

We've had rapid urine tests, urine cultures, urine calcium test, kidney ultrasound, and abdominal x-ray. We are probably going to visit a pediatric urologist next. Does anyone have any insight?