What Causes Blood In Urine After Undergoing A Limited TURP?

I had a limited TURP 3 weeks ago (to remove a lobe of the prostate growing across the area my bladder exits to the uthrea) which I was told I needed before I could have prostate brackytherapy. I still have some blood in the urine when I start peeing. Wondered when masturbation is possible without affecting the success of the surgery?