It's not uncommon to have trace amounts of blood (or red blood cells) in the urine. Certain people are just "secreters". I found out that I'm a secreter when we did urine testing in my anatomy/physiology class several years ago. My urine showed positive for blood, so I went to the doctor to get tested. He referred me to a urologist, then told me I most likely had kidney or bladder cancer and would die (no, I'm not kidding). I left the office in tears, but after a visit to the urologist, found out I was just fine, and that this is fairly common, and nothing to be concerned about.
Needless to say, I never went back to that doctor again! Follow up with all recommended tests, but don't stress...it's probably nothing.