Persistent flank pain is a sign of upper urinary tract infection until proven otherwise. Such a pain, along with the plethora of symptoms that you are experiencing (chills, fever, malaise, urinary frequency) it is almost impossible to miss this diagnosis. It is called Acute Pyelonephritis and should be treated immediately since local and general complications may arise, specially if a woman and pregnant. Urine culture and prompt treatment is necessary, it usually takes 10-14 days of proper antibiotic use to clear the infection. Urinary stones can be ruled out if suspected since they might be related to the infection. In your case they performed a very expensive test (Uro-TAC) but failed to perform a much simpler and useful test, a simple urine test and its culture. Seek medical attention again