Urinalysis: trace of protein, 1+ bilirubin, trace leukocytes. Had blood work done last couple of months, everything seemed fine. What could it be?

I wouldn't worry. Trace protein is never worth worrying about. If you're a female, the leukocytes are probably a vaginal contaminant. 1+ bilirubin is a lab error if your serum conjugated / direct bilirubin is not elevated.