What Causes Burning Sensation In Anus After Taking Chemotherapy For Colon Cancer?

Hi... I've had a burning anus for over a week. I recently had surgery to remove a tumor in my sigmoid colon, diagnosis colon cancer, and I did my first session of chemo 10 days ago... I was constipated and must have done some damage, but i've rarely had problems like this before. I went to my doctor who said it was a hemorrhoid, and he gave me some Daflon tablets and some suppositories. It seemed to get a little better, but it is still quite persistent. I will have a follow up tomorrow and my 2nd chemo session is a week from today. I want this to be cured before I do the next chemo. Why is it burning so much? Do the chemo drugs irritate it more and prevent it from healing quickly?