If a person takes an orally administered bowel cancer treatment drug, how long will it take the doctor be able to know if the drug is working or not, and if not, change the medication?

I can't feel anything on the exterior of my anus but I have blood on the toilet paper sometimes.. my anus is not scratched so is there any other explanation other than an internal hemorrhoid?
and if it is so.. is there any natural treatment or an over the counter medication i can use?