What Causes Pressure In Rectal Area?

I have been having a feeling of pressure in my rectal area as if I need to defecate. I also have mild, dull pain in my lower back. I can urinate with no trouble and there is no blood in my stool. I have had this before and a rectal exam by a urologist (about a year ago) determined that my prostate was slightly enlarged , but the lab work showed nothing unusual and he did not seem at all concerned. He sent me on my way with some Cialis samples. This seems to come and go, usually occurring at least once a year on average. I am 45 years old, about 175 pounds and I am a runner so I am in better than average condition. I do sit much of the day at work. I have had, for some time now, problems with ED, but I seem to have normal erections (sadly only) while sleeping. Any advice would be appreciated.