Sir. ultasound found stone5mm in left kidney,and there is mild fullness of b/l renal pelvi-calyeeal system in ultrasound study,no infection in urine test. sharp pain in left kidney and dull ach pain in right kidney.what cause pain in right kidney?

Because there is bilateral pelvocaliectasis, even mild, and you have right flank/back (kidney) pain with the presence of a stone in the left kidney, I think a CT scan should be done for ruling out the right ureteral stone which can't be detected by ultrasound. Mild, congenital ureteropelvic junction obstruction, if it is true as showed by ultrasound, is usually asymptomatic.