What Causes Lower Abdominal Pain Accompanied By Slight Pehile Bleeding?

I am a 34 year old 5'10" 188lb male with polycythemia vera (the only medical history I really have). I have now found blood in the front of my underwear twice in the last month. I do have lower abdominal pain just for a minute about once a month. There is no blood in my urine and I haven't experienced pain on the days I have found the blood. I am married and monogomous and I'm completely clueless on what is going on.