What Causes Blood In Urine?

Question : about 4 years ago I had slight red blood in my urine that only lasted less than one day any only two times. I went to a urologist who did a cystoscopy and a MRI and could not find anything wrong. A year and a half later May of 2012 I again had red blood in my urine that only lasted less than one day and only a couple of time. I was in a different state then and I had a urologist do the same procedures who also could not find anything except he told me that the veins around the prostrate was engorged. Then about a year and a half later December 2013 I had the same incident again and for the same period just one day and just a couple of times. However this time the blood droplets looked coagulated , just one or two . Then in February of this year 2014 it happened again and now today April 2nd it happened again. All this time I haved no other symptoms no frequent urination, no pain. I am reluctant to go back to the urologist because I believe they will not find anything again. Is it possible this is caused from leaking of the vein(s) around the Prostate. I am currently 71 retired and in excellent health working out 3/4 times a week and I am previously a marathoner