Hi. My Daughter Had An Ultrasound A Month Ago That

Hi. My daughter had an ultrasound a month ago that showed a 1.7cm "cyst" lower pole left kidney, representing either a peripelvic cyst or focal lower moiety hydronephrosis. She had a repeat ultrasound today and it showed there is cystic structure noted in the lower left kidney which is probably a mild fullness of the left renal pelvis/collecting system rather than parapelvic cyst.
Her pediatrician stated that it is nothing to be concerned about. I however disagree. My daughter has been complaining of left flank pain on and off for the past few weeks. So I called a pediatric urologist and scheduled an appt.
Could you please tell me what the fullness means