During the last half of pregnancy dilation of the upper urinary tract can occur due to compression of the ureters by the growing fetus/uterus and the linea terminalis. The compression of these is not allowing fluid passage into the bladder causing urine buildup in the kidney, this leads to dilation of the kidney that ends up causing symptomatic hydronephrosis. this can end up in infection and symptoms such as the ones present in this patient like back pain and CVA tenderness. the crying is mainly due to the pain caused by the obstruction.
and just as something extra: double pigtail catheters is a safe and simple way of treating symptomatic hydronephrosis of pregnancy.