What Does Blood In Semen Indicate?

I haven t had sex for several years now, or even erections. However, my anticipating a better future lately, I began experimenting with self-stimulation again 2 weeks ago., and noticed pink semen, although of low quantity. I did so again 2 days ago, and found actual red blood, a significant amount. I have bladder cancer from 3 years ago, but sarcoma removal at stage 1 in Nov. 2014, plus BCG treatments plus scope exams every 3 months since, plus pelvic MRI w/contrast Dec. 2016 are all negative. What is possible cause of this now? My PSA from last week is 0.8, historically it s been 0.3. Thank you for your interest in my case.