What Does A Mass In The Lower Bowel Indicate?

I was recently diagnosed with a mass in my lower bowel pressing on to my bladder, the mass is 10cm in lenth, I am due to have an sigmoidoscopy,. I have had this slight pain for a number of months now and have dirarrea there has been no dark or crimson like blood in my stools, but recently there was alot of bright red blood came out this has happened twice over a six month period. I also had a CT scan that showed no problems with any other organs at all lung, spleen, kidneys, liver. all are perfectly normal and no fluid around them, my question is could this be a benign tumour my GP tells me no matter what it can be shrinked and removed. thankyou