i doubt it's bowel cancer, it sounds more like a localised irritation! bowel cancer is indicated more by changes in your 'movements' such a blood in them. (correct me if i am wrong)
i'm sure it could be piles or some kind ir infection in the skin, maybe anal fissures?
try googling haemorroids (i think that's how you spell it) or piles, or anal fissures and see if then match your symptoms. you should try using a little mirror to get a visual picture as well...none of these are serious conditions by the way!
you can get creams etc over the counter to sort them out.
if you are really worried the doc is the best place to go. and dont worry, he has seen a thousand asses.