Yes. Flank pain in the setting of a urinary tract infection could indicate ascending infection from the bladder to the kidney. Fever is a common sign of a kidney infection. The urine should be cultured for the identity of the pathogen and it's sensitivity determined. Appropriate antibiotics that cover kidney infections should be started empirically, then tailored to the results of the urine culture.
?pyelonephritis. Sensible concern is ok, try never to worry. Flank pain with UTI suggests that the infection has reached the kidney and will require treatment to last longer & with an antibiotic that achieves good/therapeutic tissue levels. Kidney stone, bladder reflux and/or partial blockage (uretero-pelvic junction obstruction) are also possibilities. Good luck.