What Causes Blood In Urine After Retrograde Ejaculation?

I had a TURP 15 years ago , but about 1 year ago after having sex and retrograde ejaculation with no emission of ejaculate from the penis. Then upon going to urinate there is a bit of blood in the very beginning with no more after empting the bladder the only time this occurs is after my retrograde ejaculations what could be the causes. Thanks I am 72 years of age