What could cause mild blood in urine for more than 1 and a half months?

Cancer. Need to be checked, very important to rule out cancer.
See a urologist. Blood in the urine could be caused by many things- the most common are- trauma, urinary tract infection , and stones in the urinary tract. However, cancer in the urinary tract such as bladder cancer, kidney cancer, etc-also commonly are presented with blood in the urine. Considering you have this problem for 6 weeks- you should see a urologist to have further evaluation- ct scan, cystoscopy etc.