Should I be worried about a trace amount of blood in my urine and fairly frequent urination?

i'm a 29 year old male. traces of blood were found in a urine examination after a routine physical, and then again at a urologist's office. i'm in no pain whatsoever and i have never seen anything but perfectly clear urine when i go to the bathroom, but i do urinate fairly frequently (also drink a good amount of water). the urologist said he's not at all worried about it, but i have to go back for a renal sonogram (already had one on the bladder, which was normal), and frankly, i'm a worrier. should i be concerned about anything major, like cancer??