Hi, I don't know if you have heard from your doc or not, but I wanted to let you know my experience w/ blood in urine and kidney ultrasounds. I have had so many kidney stones in my life and so much treatment for them that I have forgotten how many now. If you have strep throat, there may be a related infection in your urinary tract. Stones can harbor infection and cause blood in the urine. I know from experience that it is possible to have a stone and/or infection without having severe symtoms, at least in the beginning. I am assuming that your doc is also doing a culture to check for infection.
If you have the ultrasound and the radiologist says things look ok, follow up with you doc about the culture and check again for blood. If the blood is still there after your course of antibiotics, then you should ask to be referred ro a nephrologist. They are specialist in the functions of the kidney. A urologist would be recommended if there is a stone or an infection that doesn't clear up with adequate antibiotic treatment.
Please try not to worry TOO much. There are many reasons why you would have blood in the urine and many treatment options once the problems have been diagnosed. Good luck and please let me know how things turn out.