I'm Aliza, a BC patient. I'm also a retired Medical Librarian who offers referrals and research for people on this site and elsewhere. I'm familiar to some degree with Colon cancer because there are many people in my family who've had it (no one had BC [I'm a pioneer...;)]). It seems like a logical move to me that your Oncologist should be referring you to a Proctologist if she/he knows what you're experiencing. That's my best idea for you. I'm sure there are other Colon Cancer patients or medical professionals on the site with better ideas than mine.
Wishing you a speedy recovery from your discomfort,