Could I have kidney failure if the ultrasound was normal?

Yes. The kidney is responsible for excreting waste and maintaining salt and water balance among many other functions. These are performed by special cells with enzymes/proteins within the kidney at an ultrastructural level the function of the kidney may be reduced even if the ultrasound is normal. Ultrasounds are useful to detect size and shape of kidneys and see if the urine flow is blocked.
Definitely. An ultrasound only shows the structure of the kidney. It can tell whether there is a kidney stone or an something obstructuing urine from leaving the kidney. Kidney failure can still occur even if an ultrasound is perfectly normal. Kidney failure can also be diagnosed with blood and urine tests as well.