Hi, I Am A 50 Year Old Gay Male. I

Hi, i am a 50 year old gay male. I am HIV negative as of 9/16/16 and i am on Truvada PreP. I am not as safe with condom use as i should be. That being said, i have been having symptoms in the lower GI tract that include an increase in gas and a pain and burning in my anus. My stools often have a whitish mucus looking attachment to the beginning of the stool. There has been bright colored blood both in the stool and on the paper. There is always a wetness and mucus like residue on the paper as well. These symptoms have been present for 2 months. I had a colonoscopy one week ago and i know that a polyp was removed and biopsied and i haven't received the results form that test as of yet. What is causing these symptoms and how to i test for a problem like an STD? Will the routine colonoscopy detect a problem like an STD if it is intact what this is?