Why would dr ask for a kidney ultrasound after i had already had a ct with contrast scan in the same area?

Cant think of many reasons to do that unless they want to see how big your bladder is after you pee. Discuss with your Drs . They should have better idea than I will ever have.

The characteristics of renal parenchyma (the structural part of the kidney which is directly related to kidney function) are better portrayed by ultrasound than CT scan.

Kidney CT with contrast should not be followed by ultra sound unless there is a very specific reason (not many) so ask your MD -why?. In the future please remember to ask ordering physician the reason for the test and how it will help in the diagnosis and treatment and what is involved in doing it.